| Cover | Jan. 29, 2020 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Jan. 29, 2020 |
| Entity Registrant Name | FIRST INTERSTATE BANCSYSTEM, INC. |
| Entity Incorporation, State or Country Code | MT |
| 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 | 59116-0918 |
| City Area Code | (406) |
| Local Phone Number | 255-5390 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Class A common stock, no par value |
| Trading Symbol | FIBK |
| Security Exchange Name | NASDAQ |
| Amendment Flag | false |
| Entity Central Index Key | 0000860413 |
| Entity Emerging Growth Company | false |
