Acquisitions - Schedule of Acquired Loans not Deemed to Have Credit Impairment (Details) $ in Millions	May 30, 2017 USD ($)
Business Combinations [Abstract]
Contractually required principal and interest payments	$ 2,098.1
Contractual cash flows not expected to be collected due to projected prepayment	23.3
Fair value at acquisition	$ 2,066.5

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