Acquisitions Schedule of Acquired Loans not Deemed to Have Credit Impairment (Details) $ in Thousands	Aug. 12, 2016 USD ($)
Business Combinations [Abstract]
Loans Acquired in Transfer, Contractually Required Payments Receivable at Acquisition	$ 100,616
Loans Acquired in Transfer, Nonaccretable Difference	4,702
Loans Acquired in Transfer at Acquisition Fair Value	$ 76,136

Loans Acquired in Transfer, Contractually Required Payments Receivable at Acquisition

No definition available.

Name:	fibk_LoansAcquiredinTransferContractuallyRequiredPaymentsReceivableatAcquisition
Namespace Prefix:	fibk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant