XML 72 R72.htm IDEA: XBRL DOCUMENT
v3.20.1

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2020 | Apr. 29, 2020 | Mar. 27, 2020 | Dec. 31, 2019
Subsequent Event [Line Items] |  |  |  |
Loans and Leases Receivable, Gross | $ 9,012.4 |  | $ 1,000.0 | $ 9,031.6
Financing Receivable, Troubled Debt Restructuring, Premodification | 1,100.0 |  |  |
Financing Receivable, Modifications, Recorded Investment, Non Accrual | 17.4 |  |  | $ 19.4
Class A Common Stock | Subsequent Event |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Dividend amount per share (in dollars) |  | $ 0.34 |  |
Mortgages [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Financing Receivable, Modifications, Recorded Investment, Non Accrual | $ 46.6 |  |  |

X
- Definition Financing Receivable, Modifications, Recorded Investment, Non Accrual + References No definition available. + Details
Name: | fibk_FinancingReceivableModificationsRecordedInvestmentNonAccrual
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The per share amount of a dividend declared, but not paid, as of the financial reporting date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4304-108586 + Details
Name: | us-gaap_DividendsPayableAmountPerShare
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount, before troubled debt restructuring, of financing receivable to be modified. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524 + Details
Name: | us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522 + Details
Name: | us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_MortgagesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |