XML 22 R12.htm IDEA: XBRL DOCUMENT
v3.20.2

Loans Held for Sale | 6 Months Ended
--- | ---
Jun. 30, 2020
Loans Held for Sale [Abstract] |
Loans Held for Sale | Loans Held for Sale Mortgage loans held for immediate sale in the secondary market were $169.9 million as of June 30, 2020, compared to $100.9 million as of December 31, 2019. Residential loans that the Company originated with the intent to sell are recorded at fair value. Conforming agency mortgage production is sold on a servicing retained basis. Certain loans, such as government guaranteed mortgage loans are sold on a servicing released basis.

X
- Definition Loans Held For Sale [Text Block] + References No definition available. + Details
Name: | fibk_LoansHeldForSaleTextBlock
Namespace Prefix: | fibk_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Loans Held for Sale [Abstract] + References No definition available. + Details
Name: | fibk_LoansHeldforSaleAbstract
Namespace Prefix: | fibk_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration