{"cik":860413,"company":{"entity_status":null,"filings":[{"accession":"0000860413-26-000097","document":"fibk-20261022.htm","filed":"2026-10-22","form":"8-K","items":"2.02,7.01,8.01,9.01","report_date":"2026-10-22"},{"accession":"0001493152-26-045898","document":"xslF345X06/ownership.xml","filed":"2026-10-06","form":"4","items":"","report_date":"2026-10-05"},{"accession":"0002090710-26-000006","document":"xslF345X06/wk-form4_1790115658.xml","filed":"2026-09-22","form":"4","items":"","report_date":"2026-09-10"},{"accession":"0002154828-26-000005","document":"xslF345X06/wk-form4_1789578965.xml","filed":"2026-09-16","form":"4","items":"","report_date":"2026-09-14"},{"accession":"0001493152-26-042992","document":"xslF345X06/ownership.xml","filed":"2026-09-16","form":"4","items":"","report_date":"2026-09-16"},{"accession":"0000860413-26-000092","document":"fibk-20260914.htm","filed":"2026-09-16","form":"8-K","items":"5.02,9.01","report_date":"2026-09-14"},{"accession":"0001493152-26-042633","document":"xslF345X06/ownership.xml","filed":"2026-09-14","form":"4","items":"","report_date":"2026-09-14"},{"accession":"0001493152-26-042536","document":"xslF345X06/ownership.xml","filed":"2026-09-14","form":"4","items":"","report_date":"2026-09-11"},{"accession":"0000860413-26-000088","document":"fibk-20260821.htm","filed":"2026-08-21","form":"8-K","items":"5.02,9.01","report_date":"2026-08-21"},{"accession":"0000860413-26-000084","document":"fibk-20260630.htm","filed":"2026-08-03","form":"10-Q","items":"","report_date":"2026-06-30"},{"accession":"0001493152-26-035055","document":"xslF345X06/ownership.xml","filed":"2026-07-28","form":"4","items":"","report_date":"2026-07-27"},{"accession":"0002146309-26-000004","document":"xslF345X06/wk-form4_1784932270.xml","filed":"2026-07-24","form":"4","items":"","report_date":"2026-07-22"},{"accession":"0000860413-26-000081","document":"fibk-20260723.htm","filed":"2026-07-23","form":"8-K","items":"2.02,7.01,8.01,9.01","report_date":"2026-07-23"},{"accession":"0000860413-26-000078","document":"fibk-20260713.htm","filed":"2026-07-16","form":"8-K","items":"5.02,9.01","report_date":"2026-07-13"},{"accession":"0000860413-26-000075","document":"fibk-20260709.htm","filed":"2026-07-09","form":"8-K","items":"5.02,9.01","report_date":"2026-07-09"},{"accession":"0000860413-26-000048","document":"fibk-20260527.htm","filed":"2026-05-29","form":"8-K","items":"5.02,5.03,5.07,9.01","report_date":"2026-05-27"},{"accession":"0000860413-26-000041","document":"fibk-20260511.htm","filed":"2026-05-13","form":"8-K","items":"5.02,9.01","report_date":"2026-05-11"},{"accession":"0000860413-26-000038","document":"fibk-20260331.htm","filed":"2026-05-07","form":"10-Q","items":"","report_date":"2026-03-31"},{"accession":"0000860413-26-000034","document":"fibk-20260429.htm","filed":"2026-04-29","form":"8-K","items":"2.02,7.01,8.01,9.01","report_date":"2026-04-29"},{"accession":"0000860413-26-000025","document":"fibk-20260415.htm","filed":"2026-04-16","form":"DEF 14A","items":"","report_date":"2025-12-31"},{"accession":"0000860413-26-000011","document":"fibk-20251231.htm","filed":"2026-02-26","form":"10-K","items":"","report_date":"2025-12-31"},{"accession":"0000860413-26-000007","document":"fibk-20260128.htm","filed":"2026-01-28","form":"8-K","items":"2.02,7.01,8.01,9.01","report_date":"2026-01-28"},{"accession":"0000860413-25-000168","document":"fibk-20251114.htm","filed":"2025-11-18","form":"8-K","items":"5.02,9.01","report_date":"2025-11-14"},{"accession":"0000860413-25-000165","document":"fibk-20250930.htm","filed":"2025-11-06","form":"10-Q","items":"","report_date":"2025-09-30"},{"accession":"0000860413-25-000160","document":"fibk-20251029.htm","filed":"2025-10-29","form":"8-K","items":"2.02,7.01,8.01","report_date":"2025-10-29"},{"accession":"0000860413-25-000155","document":"fibk-20251016.htm","filed":"2025-10-16","form":"8-K","items":"7.01,9.01","report_date":"2025-10-16"},{"accession":"0000860413-25-000152","document":"fibk-20251010.htm","filed":"2025-10-14","form":"8-K","items":"7.01,9.01","report_date":"2025-10-10"},{"accession":"0000860413-25-000147","document":"fibk-20250903.htm","filed":"2025-09-09","form":"8-K","items":"5.02,7.01,9.01","report_date":"2025-09-03"},{"accession":"0000860413-25-000137","document":"fibk-20250829.htm","filed":"2025-09-02","form":"8-K","items":"5.02,9.01","report_date":"2025-08-29"},{"accession":"0000860413-25-000135","document":"fibk-20250813.htm","filed":"2025-08-29","form":"8-K/A","items":"5.02","report_date":"2025-08-13"},{"accession":"0000860413-25-000129","document":"fibk-20250828.htm","filed":"2025-08-28","form":"8-K","items":"8.01,9.01","report_date":"2025-08-28"},{"accession":"0000860413-25-000122","document":"fibk-20250815.htm","filed":"2025-08-18","form":"8-K","items":"1.02,8.01,9.01","report_date":"2025-08-15"},{"accession":"0000860413-25-000119","document":"fibk-20250813.htm","filed":"2025-08-15","form":"8-K","items":"5.02,9.01","report_date":"2025-08-13"},{"accession":"0000860413-25-000117","document":"fibk-20250630.htm","filed":"2025-08-06","form":"10-Q","items":"","report_date":"2025-06-30"},{"accession":"0000860413-25-000114","document":"fibk-20250729.htm","filed":"2025-07-29","form":"8-K","items":"2.02,7.01,8.01,9.01","report_date":"2025-07-29"},{"accession":"0001104659-25-065784","document":"tm2519885d1_8k.htm","filed":"2025-07-03","form":"8-K","items":"8.01,9.01","report_date":"2025-07-03"},{"accession":"0001104659-25-058170","document":"tm258437d8_8k.htm","filed":"2025-06-10","form":"8-K","items":"1.01,2.03,9.01","report_date":"2025-06-10"},{"accession":"0001104659-25-057359","document":"tm258437d7_8k.htm","filed":"2025-06-06","form":"8-K","items":"7.01,8.01,9.01","report_date":"2025-06-05"},{"accession":"0000860413-25-000078","document":"fibk-20250602.htm","filed":"2025-06-02","form":"8-K","items":"7.01,9.01","report_date":"2025-06-02"},{"accession":"0000860413-25-000077","document":"fibk-20250601.htm","filed":"2025-06-02","form":"8-K","items":"5.02,7.01,9.01","report_date":"2025-06-01"}],"former_names":[{"from":"1996-12-27T05:00:00.000Z","name":"FIRST INTERSTATE BANCSYSTEM OF MONTANA INC","to":"1999-03-09T05:00:00.000Z"}],"gleif_parents":[],"headquarters":[{"latitude":45.7833,"longitude":-108.512,"name":"First Interstate Center","precision":"headquarters locality","source":"wikidata","url":"https://www.wikidata.org/wiki/Q5453110"}],"insiders":[{"accession":"0001493152-26-042992","acquired_disposed":"D","code":"S","date":"2026-09-16","derivative":false,"filed":"2026-09-16","following_shares":"484309.0","form":"4","owners":["Elizabeth Lauren Scott Rose Trust","IXL Ltd Liability Co","Juliana Sarah Scott Rose Trust","Julie A Scott Rose Trustee of the Julie A Scott Rose Trust Dated 5-14-2002","SCOTT JULIE A"],"plan_10b5_1":false,"price":"37.64","security":"Common Stock","shares":"2700.0"},{"accession":"0002154828-26-000005","acquired_disposed":"A","code":"A","date":"2026-09-14","derivative":false,"filed":"2026-09-16","following_shares":"5417.0","form":"4","owners":["Lee Jeffery D."],"plan_10b5_1":false,"price":"0.0","security":"Common Stock","shares":"5417.0"},{"accession":"0001493152-26-042633","acquired_disposed":"D","code":"S","date":"2026-09-14","derivative":false,"filed":"2026-09-14","following_shares":"817738.0","form":"4","owners":["Harper Grace Scott Trust","Harrison William Scott Trust","Holland Elizabeth Scott Trust","IXL Ltd Liability Co","Jonathan Scott as Trustee of the Jonathan R Scott Trust Dated as of 4/21/04","Scott Jonathan R"],"plan_10b5_1":false,"price":"37.29","security":"Common Stock","shares":"8071.0"},{"accession":"0001493152-26-042536","acquired_disposed":"D","code":"S","date":"2026-09-11","derivative":false,"filed":"2026-09-14","following_shares":"825809.0","form":"4","owners":["Harper Grace Scott Trust","Harrison William Scott Trust","Holland Elizabeth Scott Trust","IXL Ltd Liability Co","Jonathan Scott as Trustee of the Jonathan R Scott Trust Dated as of 4/21/04","Scott Jonathan R"],"plan_10b5_1":false,"price":"37.03","security":"Common Stock","shares":"16929.0"},{"accession":"0002090710-26-000006","acquired_disposed":"D","code":"F","date":"2026-09-10","derivative":false,"filed":"2026-09-22","following_shares":"13018.0","form":"4","owners":["Shepler Christopher L."],"plan_10b5_1":false,"price":"36.88","security":"Common Stock","shares":"222.0"},{"accession":"0001493152-26-035055","acquired_disposed":"D","code":"G","date":"2026-07-27","derivative":false,"filed":"2026-07-28","following_shares":"487009.0","form":"4","owners":["Elizabeth Lauren Scott Rose Trust","IXL Ltd Liability Co","Juliana Sarah Scott Rose Trust","Julie A Scott Rose Trustee of the Julie A Scott Rose Trust Dated 5-14-2002","SCOTT JULIE A"],"plan_10b5_1":false,"price":"0.0","security":"Common Stock","shares":"1350.0"},{"accession":"0002146309-26-000004","acquired_disposed":"A","code":"A","date":"2026-07-22","derivative":false,"filed":"2026-07-24","following_shares":"1806.0","form":"4","owners":["Ritter Matthew John"],"plan_10b5_1":false,"price":"38.97","security":"Common Stock","shares":"1806.0"},{"accession":"0001196952-26-000004","acquired_disposed":"A","code":"A","date":"2026-07-22","derivative":false,"filed":"2026-07-24","following_shares":"1806.0","form":"4","owners":["TURNER BRIAN KEVIN"],"plan_10b5_1":false,"price":"38.97","security":"Common Stock","shares":"1806.0"},{"accession":"0001493152-26-033609","acquired_disposed":"D","code":"S","date":"2026-07-16","derivative":false,"filed":"2026-07-16","following_shares":"842738.0","form":"4","owners":["Harper Grace Scott Trust","Harrison William Scott Trust","Holland Elizabeth Scott Trust","IXL Ltd Liability Co","Jonathan Scott as Trustee of the Jonathan R Scott Trust Dated as of 4/21/04","Scott Jonathan R"],"plan_10b5_1":false,"price":"40.3","security":"Common Stock","shares":"10000.0"}],"legal_name":null,"lei":null,"lei_status":null,"lei_updated_at":null,"names":[{"entity_id":null,"kind":"alias","name":"First Interstate Bank","source":"wikidata"},{"entity_id":null,"kind":"alias","name":"First Interstate BancSystem","source":"wikidata"},{"entity_id":null,"kind":"alias","name":"First Interstate","source":"wikidata"}],"parent_exceptions":[],"parents":[],"wikidata_id":"Q5453107"},"research_built_at":"2026-10-23T06:11:20.354311+00:00","reviewed_events":[],"reviewed_events_coverage":"selective; not a complete event history; accounting series not reconciled","sources":[{"coverage":"8022 issuers; up to 32 recent company reports and 8 ownership-change filings per issuer","id":"sec","license":"SEC public-information reuse","license_url":"https://www.sec.gov/about/privacy-information","name":"SEC submissions","retrieved_at":"2026-10-23T06:12:00.336203+00:00","status":"ok","url":"https://www.sec.gov/search-filings/edgar-application-programming-interfaces"},{"coverage":"1658 unambiguous CIK-linked entities; coverage is incomplete","id":"wikidata","license":"CC0-1.0","license_url":"https://www.wikidata.org/wiki/Wikidata:Licensing","name":"Wikidata structured data","retrieved_at":"2026-10-23T06:12:25.714432+00:00","status":"ok","url":"https://www.wikidata.org/"},{"coverage":"1197 checked identities; 0 unavailable records","id":"gleif","license":"CC0-1.0","license_url":"https://www.gleif.org/en/meta/lei-data-terms-of-use","name":"GLEIF legal entity reference data","retrieved_at":"2026-10-23T06:14:03.968440+00:00","status":"ok","url":"https://www.gleif.org/en/lei-data/gleif-api"},{"coverage":"4 selected series; gaps remain missing","id":"fed","license":"Public domain (selected government series)","license_url":"https://www.federalreserve.gov/disclaimer.htm","name":"Federal Reserve Board H.15","retrieved_at":"2026-10-23T06:27:13.562713+00:00","status":"ok","url":"https://www.federalreserve.gov/releases/h15/"},{"coverage":"9 selected series; gaps remain missing","id":"bls","license":"Public domain","license_url":"https://www.bls.gov/opub/copyright-information.htm","name":"Bureau of Labor Statistics","retrieved_at":"2026-10-23T06:27:14.878114+00:00","status":"ok","url":"https://www.bls.gov/developers/"},{"coverage":"2026 Q3: filings in this completed quarter; not a live transaction feed","id":"insiders","license":"SEC public-information reuse","license_url":"https://www.sec.gov/about/privacy-information","name":"SEC insider transactions","retrieved_at":"2026-10-23T06:12:21.860189+00:00","status":"ok","url":"https://www.sec.gov/data-research/sec-markets-data/insider-transactions-data-sets"}]}