Loans Schedule of Loans Acquired from Business Combinations (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities [Abstract]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Outstanding Balance	$ 42,627
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Carrying Amount Net, Accrual Status	32,350
Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Carrying Amount Net, Non Accrual Status	0
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Carrying Amount, Net	$ 32,350

Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Carrying Amount Net, Accrual Status

No definition available.

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

- Definition

Certain Loans Acquired In Transfer Not Accounted For As Debt Securities Carrying Amount Net, Non Accrual Status

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 15

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

+ Details

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

+ Details

Name:	us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesOutstandingBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant