Cover |
Dec. 31, 2026 |
May 27, 2026 |
|---|---|---|
| Cover [Abstract] | ||
| Document Type | 8-K | |
| Document Period End Date | May 27, 2026 | |
| Entity Registrant Name | FIRST INTERSTATE BANCSYSTEM, INC. | |
| Entity Incorporation, State or Country Code | DE | |
| Entity File Number | 001-34653 | |
| Entity Tax Identification Number | 81-0331430 | |
| Entity Address, Address Line One | 401 North 31st Street | |
| Entity Address, City or Town | Billings, | |
| Entity Address, State or Province | MT | |
| Entity Address, Postal Zip Code | 59101 | |
| City Area Code | (406) | |
| Local Phone Number | 255-5311 | |
| Written Communications | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Title of 12(b) Security | Common stock, $0.00001 par value | |
| Trading Symbol | FIBK | |
| Security Exchange Name | NASDAQ | |
| Amendment Flag | false | |
| Entity Central Index Key | 0000860413 | |
| Entity Emerging Growth Company | false | |
| Current Fiscal Year End Date | --12-31 |
| X | ||||||||||
- Definition Boolean flag that is true when the XBRL content amends previously-filed or accepted submission. No definition available.
|
| X | ||||||||||
- Definition Area code of city No definition available.
|
| X | ||||||||||
- Definition Cover page. No definition available.
|
| X | ||||||||||
- Definition End date of current fiscal year in the format --MM-DD. No definition available.
|
| X | ||||||||||
- Definition For the EDGAR submission types of Form 8-K: the date of the report, the date of the earliest event reported; for the EDGAR submission types of Form N-1A: the filing date; for all other submission types: the end of the reporting or transition period. The format of the date is YYYY-MM-DD. No definition available.
|
| X | ||||||||||
- Definition The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'. No definition available.
|
| X | ||||||||||
- Definition Address Line 1 such as Attn, Building Name, Street Name No definition available.
|
| X | ||||||||||
- Definition Name of the City or Town No definition available.
|
| X | ||||||||||
- Definition Code for the postal or zip code No definition available.
|
| X | ||||||||||
- Definition Name of the state or province. No definition available.
|
| X | ||||||||||
- Definition A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Indicate if registrant meets the emerging growth company criteria. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Commission file number. The field allows up to 17 characters. The prefix may contain 1-3 digits, the sequence number may contain 1-8 digits, the optional suffix may contain 1-4 characters, and the fields are separated with a hyphen. No definition available.
|
| X | ||||||||||
- Definition Two-character EDGAR code representing the state or country of incorporation. No definition available.
|
| X | ||||||||||
- Definition The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Local phone number for entity. No definition available.
|
| X | ||||||||||
- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Title of a 12(b) registered security. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Name of the Exchange on which a security is registered. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as soliciting material pursuant to Rule 14a-12 under the Exchange Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Trading symbol of an instrument as listed on an exchange. No definition available.
|
| X | ||||||||||
- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as written communications pursuant to Rule 425 under the Securities Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|