XML 57 R47.htm IDEA: XBRL DOCUMENT
v3.20.2

Loans Held for Sale - Narrative (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Loans Held for Sale [Abstract] |  |
Mortgage loans held for sale | $ 102.0 | $ 100.9

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

X
- Definition Fair value portion of mortgage loans held-for-sale. + References No definition available. + Details
Name: | us-gaap_MortgagesHeldForSaleFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant