XML 88 R67.htm IDEA: XBRL DOCUMENT
v3.22.0.1

Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2021 | Dec. 31, 2018
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Total Recorded Investment | $ 43,500,000 |  |  |  |
Related Allowance | 0 |  |  |  |
Average Recorded Investment |  | $ 64,700,000 |  |  |
Accounts Receivable, Allowance for Credit Loss | 122,300,000 | 144,300,000 | $ 73,000,000.0 | $ 144,300,000 | $ 73,000,000.0
Provision charged (credited) to operating expense | (14,600,000) | 56,900,000 | 13,900,000 |  |
Less loans charged-off |  |  | (23,600,000) |  |
Add back recoveries of loans previously charged-off |  |  | 9,700,000 |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 11,700,000 | 17,500,000 |  |  |
Individually evaluated for impairment |  |  | 3,600,000 |  |
Collectively evaluated for impairment |  |  | 69,400,000 |  |
Loans and Leases Receivable, Allowance | 122,300,000 | 144,300,000 | 73,000,000.0 |  |
Individually evaluated for impairment |  |  | 64,700,000 |  |
Collectively evaluated for impairment |  |  | 8,871,500,000 |  |
Loans And Leases Receivable Held For Investment, Evaluated for Impairment |  |  | 8,936,200,000 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 300,000 |  |  |
Loans and Leases Receivable, Allowance | 9,331,700,000 | 9,807,500,000 |  |  |
Accounts and Financing Receivable, after Allowance for Credit Loss, Noncurrent | 100,000 |  |  |  |
Business Assets Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 3,000,000.0 | 7,400,000 |  |  |
Real Property Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 8,700,000 | 8,600,000 |  |  |
Other Property |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 0 | 1,500,000 |  |  |
Real Estate |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 28,900,000 |  | 31,000,000.0
Provision charged (credited) to operating expense |  |  | (1,300,000) |  |
Less loans charged-off |  |  | (3,500,000) |  |
Add back recoveries of loans previously charged-off |  |  | 2,700,000 |  |
Individually evaluated for impairment |  |  | 1,700,000 |  |
Collectively evaluated for impairment |  |  | 27,200,000 |  |
Loans and Leases Receivable, Allowance |  |  | 28,900,000 |  |
Individually evaluated for impairment |  |  | 41,100,000 |  |
Collectively evaluated for impairment |  |  | 5,897,100,000 |  |
Loans And Leases Receivable Held For Investment, Evaluated for Impairment |  |  | 5,938,200,000 |  |
Total consumer loans |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 9,900,000 |  | 8,700,000
Provision charged (credited) to operating expense |  |  | 10,600,000 |  |
Less loans charged-off |  |  | (13,000,000.0) |  |
Add back recoveries of loans previously charged-off |  |  | 3,600,000 |  |
Individually evaluated for impairment |  |  | 0 |  |
Collectively evaluated for impairment |  |  | 9,900,000 |  |
Loans and Leases Receivable, Allowance |  |  | 9,900,000 |  |
Individually evaluated for impairment |  |  | 0 |  |
Collectively evaluated for impairment |  |  | 1,045,200,000 |  |
Loans And Leases Receivable Held For Investment, Evaluated for Impairment |  |  | 1,045,200,000 |  |
Commercial Portfolio Segment [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 32,600,000 |  | 31,300,000
Provision charged (credited) to operating expense |  |  | 4,500,000 |  |
Less loans charged-off |  |  | (6,600,000) |  |
Add back recoveries of loans previously charged-off |  |  | 3,400,000 |  |
Individually evaluated for impairment |  |  | 1,700,000 |  |
Collectively evaluated for impairment |  |  | 30,900,000 |  |
Loans and Leases Receivable, Allowance |  |  | 32,600,000 |  |
Individually evaluated for impairment |  |  | 17,300,000 |  |
Collectively evaluated for impairment |  |  | 1,656,400,000 |  |
Loans And Leases Receivable Held For Investment, Evaluated for Impairment |  |  | 1,673,700,000 |  |
Agriculture |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 1,600,000 |  | 2,000,000.0
Provision charged (credited) to operating expense |  |  | 100,000 |  |
Less loans charged-off |  |  | (500,000) |  |
Add back recoveries of loans previously charged-off |  |  | 0 |  |
Individually evaluated for impairment |  |  | 200,000 |  |
Collectively evaluated for impairment |  |  | 1,400,000 |  |
Loans and Leases Receivable, Allowance |  |  | 1,600,000 |  |
Individually evaluated for impairment |  |  | 6,300,000 |  |
Collectively evaluated for impairment |  |  | 272,800,000 |  |
Loans And Leases Receivable Held For Investment, Evaluated for Impairment |  |  | 279,100,000 |  |
Other Segments |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 0 |  | $ 0
Provision charged (credited) to operating expense |  |  | 0 |  |
Less loans charged-off |  |  | 0 |  |
Add back recoveries of loans previously charged-off |  |  | 0 |  |
Other |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Individually evaluated for impairment |  |  | 0 |  |
Collectively evaluated for impairment |  |  | 0 |  |
Loans and Leases Receivable, Allowance |  |  | 0 |  |
Individually evaluated for impairment |  |  | 0 |  |
Collectively evaluated for impairment |  |  | 0 |  |
Loans And Leases Receivable Held For Investment, Evaluated for Impairment |  |  | 0 |  |
Commercial Real Estate Portfolio Segment [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 12,900,000 |  |  |
Recorded Investment With Allowance |  | 10,800,000 |  |  |
Total Recorded Investment |  | 23,700,000 |  |  |
Related Allowance |  | 700,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 19,500,000 | 54,800,000 |
Unpaid Total Principal Balance |  | 29,200,000 |  |  |
Land Acquisition And Development Construction Financing Receivable [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 400,000 |  |  |
Recorded Investment With Allowance |  | 2,600,000 |  |  |
Total Recorded Investment |  | 3,000,000.0 |  |  |
Related Allowance |  | 500,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 1,900,000 | 1,300,000 |
Unpaid Total Principal Balance |  | 9,200,000 |  |  |
Financing Receivable, Originated in Current Fiscal Year | 113,800,000 | 105,100,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 41,800,000 | 58,900,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 34,200,000 | 27,600,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 15,400,000 | 31,700,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 20,200,000 | 7,600,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 21,200,000 | 27,700,000 |  |  |
Financing Receivable, Revolving | 1,200,000 | 6,400,000 |  |  |
Loans and Leases Receivable, Allowance | 247,800,000 | 265,000,000.0 |  |  |
Residential Real Estate Construction Financing Receivable [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 0 |  |  |
Recorded Investment With Allowance |  | 0 |  |  |
Total Recorded Investment |  | 0 |  |  |
Related Allowance |  | 0 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 1,500,000 | 1,600,000 |
Unpaid Total Principal Balance |  | 100,000 |  |  |
Financing Receivable, Originated in Current Fiscal Year | 112,400,000 | 80,600,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 7,400,000 | 64,700,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 13,700,000 | 16,700,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 900,000 | 5,600,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 400,000 | 0 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 0 | 100,000 |  |  |
Financing Receivable, Revolving | 127,200,000 | 83,200,000 |  |  |
Loans and Leases Receivable, Allowance | 262,000,000.0 | 250,900,000 |  |  |
Commercial Real Estate Construction [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 500,000 |  |  |
Recorded Investment With Allowance |  | 0 |  |  |
Total Recorded Investment |  | 500,000 |  |  |
Related Allowance |  | 100,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 2,700,000 | 7,300,000 |
Unpaid Total Principal Balance |  | 1,000,000.0 |  |  |
Financing Receivable, Originated in Current Fiscal Year | 209,700,000 | 236,100,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 141,400,000 | 195,400,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 118,800,000 | 62,700,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 27,600,000 | 11,900,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 0 | 6,000,000.0 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 500,000 | 200,000 |  |  |
Financing Receivable, Revolving | 0 | 11,600,000 |  |  |
Loans and Leases Receivable, Allowance | 498,000,000.0 | 523,500,000 |  |  |
Real Estate Construction Financing Receivable [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 900,000 |  |  |
Recorded Investment With Allowance |  | 2,600,000 |  |  |
Total Recorded Investment |  | 3,500,000 |  |  |
Related Allowance |  | 600,000 |  |  |
Unpaid Total Principal Balance |  | 10,300,000 |  |  |
Agricultural Real Estate Financing Receivable [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 5,200,000 |  |  |
Recorded Investment With Allowance |  | 3,000,000.0 |  |  |
Total Recorded Investment |  | 8,200,000 |  |  |
Related Allowance |  | 200,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 500,000 | 2,700,000 |
Unpaid Total Principal Balance |  | 8,600,000 |  |  |
Financing Receivable, Originated in Current Fiscal Year | 62,400,000 | 54,200,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 38,600,000 | 58,200,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 37,300,000 | 32,600,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 23,300,000 | 19,600,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 13,200,000 | 17,000,000.0 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 33,300,000 | 32,600,000 |  |  |
Financing Receivable, Revolving | 5,800,000 | 6,400,000 |  |  |
Loans and Leases Receivable, Allowance | 213,900,000 | 220,600,000 |  |  |
Real Estate Financing Receivable [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 22,900,000 |  |  |
Recorded Investment With Allowance |  | 18,200,000 |  |  |
Total Recorded Investment |  | 41,100,000 |  |  |
Related Allowance |  | 1,700,000 |  |  |
Average Recorded Investment |  | 41,400,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 28,900,000 | 80,500,000 |
Loans and Leases Receivable, Collateral for Secured Borrowings | 8,200,000 | 8,900,000 |  |  |
Unpaid Total Principal Balance |  | 55,000,000.0 |  |  |
Impaired Financing Receivable, Interest Income, Accrual Method |  | 100,000 |  |  |
Real Estate Financing Receivable [Member] | Business Assets Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 1,200,000 | 1,300,000 |  |  |
Real Estate Financing Receivable [Member] | Real Property Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 7,000,000.0 | 6,500,000 |  |  |
Real Estate Financing Receivable [Member] | Other Property |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 0 | 1,100,000 |  |  |
Agricultural Financing Receivable [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 2,300,000 |  |  |
Recorded Investment With Allowance |  | 4,000,000.0 |  |  |
Total Recorded Investment |  | 6,300,000 |  |  |
Related Allowance |  | 200,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 1,600,000 | 700,000 |
Loans and Leases Receivable, Collateral for Secured Borrowings | 700,000 | 800,000 |  |  |
Unpaid Total Principal Balance |  | 6,900,000 |  |  |
Agricultural Financing Receivable [Member] | Business Assets Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 0 | 0 |  |  |
Agricultural Financing Receivable [Member] | Real Property Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 700,000 | 800,000 |  |  |
Agricultural Financing Receivable [Member] | Other Property |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 0 | 0 |  |  |
Residential Real Estate |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 3,900,000 |  |  |
Recorded Investment With Allowance |  | 1,800,000 |  |  |
Total Recorded Investment |  | 5,700,000 |  |  |
Related Allowance |  | 200,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 2,800,000 | 12,800,000 |
Unpaid Total Principal Balance |  | 6,900,000 |  |  |
Commercial Portfolio Segment [Member] |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 12,000,000.0 |  |  |
Recorded Investment With Allowance |  | 5,300,000 |  |  |
Total Recorded Investment |  | 17,300,000 |  |  |
Related Allowance |  | 1,700,000 |  |  |
Average Recorded Investment |  | 18,700,000 |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 32,600,000 | 39,200,000 |
Loans and Leases Receivable, Collateral for Secured Borrowings | 2,800,000 | 7,800,000 |  |  |
Unpaid Total Principal Balance |  | 25,500,000 |  |  |
Impaired Financing Receivable, Interest Income, Accrual Method |  | 200,000 |  |  |
Commercial Portfolio Segment [Member] | Business Assets Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 1,800,000 | 6,100,000 |  |  |
Commercial Portfolio Segment [Member] | Real Property Pledged as Collateral |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 1,000,000.0 | 1,300,000 |  |  |
Commercial Portfolio Segment [Member] | Other Property |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Loans and Leases Receivable, Collateral for Secured Borrowings | 0 | 400,000 |  |  |
Loans held for investment |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Recorded Investment With No Allowance |  | 37,200,000 |  |  |
Recorded Investment With Allowance |  | 27,500,000 |  |  |
Total Recorded Investment |  | 64,700,000 |  |  |
Related Allowance |  | 3,600,000 |  |  |
Unpaid Total Principal Balance |  | 87,400,000 |  |  |
Home Equity Loan |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 1,000,000.0 | 1,400,000 |
Financing Receivable, Originated in Current Fiscal Year | 11,400,000 | 12,100,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 7,000,000.0 | 7,300,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 4,000,000.0 | 7,100,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 4,800,000 | 9,700,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 4,200,000 | 4,800,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 12,500,000 | 14,800,000 |  |  |
Financing Receivable, Revolving | 350,700,000 | 328,200,000 |  |  |
Loans and Leases Receivable, Allowance | 394,600,000 | 384,000,000.0 |  |  |
Home Equity Loan | Performing Financial Instruments |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Financing Receivable, Originated in Current Fiscal Year | 11,100,000 | 12,000,000.0 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 7,000,000.0 | 7,100,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 3,700,000 | 7,100,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 4,800,000 | 9,700,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 3,600,000 | 4,700,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 12,000,000.0 | 14,400,000 |  |  |
Financing Receivable, Revolving | 350,700,000 | 328,100,000 |  |  |
Loans and Leases Receivable, Allowance | 392,900,000 | 383,100,000 |  |  |
Home Equity Loan | Nonperforming Financial Instruments |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Financing Receivable, Originated in Current Fiscal Year | 300,000 | 100,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 0 | 200,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 300,000 | 0 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 0 | 0 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 600,000 | 100,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 500,000 | 400,000 |  |  |
Financing Receivable, Revolving | 0 | 100,000 |  |  |
Loans and Leases Receivable, Allowance | 1,700,000 | 900,000 |  |  |
Indirect consumer |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | 4,500,000 | 16,700,000 |
Financing Receivable, Originated in Current Fiscal Year | 273,100,000 | 334,600,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 209,100,000 | 188,100,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 108,700,000 | 118,000,000.0 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 64,200,000 | 74,000,000.0 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 37,100,000 | 47,700,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 45,400,000 | 42,700,000 |  |  |
Financing Receivable, Revolving | 0 | 0 |  |  |
Loans and Leases Receivable, Allowance | 737,600,000 | 805,100,000 |  |  |
Indirect consumer | Performing Financial Instruments |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Financing Receivable, Originated in Current Fiscal Year | 272,600,000 | 334,500,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 208,600,000 | 187,900,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 108,300,000 | 117,900,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 64,000,000.0 | 73,800,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 37,000,000.0 | 47,600,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 45,000,000.0 | 42,600,000 |  |  |
Financing Receivable, Revolving | 0 | 0 |  |  |
Loans and Leases Receivable, Allowance | 735,500,000 | 804,300,000 |  |  |
Indirect consumer | Nonperforming Financial Instruments |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Financing Receivable, Originated in Current Fiscal Year | 500,000 | 100,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 500,000 | 200,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 400,000 | 100,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 200,000 | 200,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 100,000 | 100,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 400,000 | 100,000 |  |  |
Financing Receivable, Revolving | 0 | 0 |  |  |
Loans and Leases Receivable, Allowance | 2,100,000 | 800,000 |  |  |
Other consumer |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss |  |  | $ 2,900,000 | $ 4,600,000 |
Financing Receivable, Originated in Current Fiscal Year | 42,600,000 | 47,200,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 27,900,000 | 29,400,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 15,000,000.0 | 28,200,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 13,400,000 | 11,900,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 5,800,000 | 5,300,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 7,600,000 | 8,600,000 |  |  |
Financing Receivable, Revolving | 16,900,000 | 20,000,000.0 |  |  |
Loans and Leases Receivable, Allowance | 129,200,000 | 150,600,000 |  |  |
Other consumer | Performing Financial Instruments |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Financing Receivable, Originated in Current Fiscal Year | 42,500,000 | 47,100,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 27,900,000 | 29,400,000 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 15,000,000.0 | 28,100,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 13,300,000 | 11,900,000 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 5,800,000 | 5,300,000 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 7,600,000 | 8,600,000 |  |  |
Financing Receivable, Revolving | 16,900,000 | 19,900,000 |  |  |
Loans and Leases Receivable, Allowance | 129,000,000.0 | 150,300,000 |  |  |
Other consumer | Nonperforming Financial Instruments |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Financing Receivable, Originated in Current Fiscal Year | 100,000 | 100,000 |  |  |
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year | 0 | 0 |  |  |
Financing Receivable, Originated Two Years before Latest Fiscal Year | 0 | 100,000 |  |  |
Financing Receivable, Originated Three Years before Latest Fiscal Year | 100,000 | 0 |  |  |
Financing Receivable, Originated Four Years before Latest Fiscal Year | 0 | 0 |  |  |
Financing Receivable, Originated Five or More Years before Latest Fiscal Year | 0 | 0 |  |  |
Financing Receivable, Revolving | 0 | 100,000 |  |  |
Loans and Leases Receivable, Allowance | $ 200,000 | 300,000 |  |  |
Agricultural Sector |  |  |  |  |
Impaired Financing Receivable, Unpaid Principal Balance |  |  |  |  |
Average Recorded Investment |  | 4,600,000 |  |  |
Impaired Financing Receivable, Interest Income, Accrual Method |  | $ 0 |  |  |

X
- Definition Loans And Leases Receivable Held For Investment, Evaluated for Impairment + References No definition available. + Details
Name: | fibk_LoansAndLeasesReceivableHeldForInvestmentEvaluatedForImpairment
Namespace Prefix: | fibk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of allowance for credit loss on accounts receivable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446 + Details
Name: | us-gaap_AllowanceForDoubtfulAccountsReceivable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The balance of financing receivables that were collectively evaluated for impairment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The balance of financing receivables that were individually evaluated for impairment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable originated more than five years prior to current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 79 -URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447 + Details
Name: | us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable originated four years prior to current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 79 -URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447 + Details
Name: | us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable originated in current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 79 -URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447 + Details
Name: | us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable originated in fiscal year prior to current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 79 -URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448 + Details
Name: | us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable originated three years prior to current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 79 -URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447 + Details
Name: | us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable originated two years prior to current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 79 -URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447 + Details
Name: | us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of financing receivable that can be withdrawn, repaid and redrawn. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 55 -Paragraph 79 -URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448 + Details
Name: | us-gaap_FinancingReceivableRevolving
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5212-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5258-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5212-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953550-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of allowance for credit losses related to recorded investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5212-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5212-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalanceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of investment in impaired financing receivables for which there is no related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953550-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in impaired financing receivables for which there is a related allowance for credit losses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953550-111524 + Details
Name: | us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB Topic 6.L.1) -URI http://asc.fasb.org/extlink&oid=122038336&loc=d3e74567-122707 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Reflects the aggregate value of loans, trade receivables, securities and financial instruments that serve as collateral for obligations arising from a transfer of financial assets recorded as a secured borrowing. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=123577603&loc=d3e5086-111524 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729 + Details
Name: | us-gaap_LoansAndLeasesReceivableCollateralForSecuredBorrowings
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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=124259787&loc=d3e4428-111522 + Details
Name: | us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount, after allowance for credit loss, of accounts and financing receivables, classified as noncurrent. Includes, but is not limited to, notes and loan receivable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(3)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermAccountsNotesAndLoansReceivableNetNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of expense related to estimated loss from loan and lease transactions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.11) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 + Details
Name: | us-gaap_ProvisionForLoanAndLeaseLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_CollateralAxis=fibk_BusinessAssetsPledgedAsCollateralMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CollateralAxis=fibk_RealPropertyPledgedAsCollateralMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CollateralAxis=srt_OtherPropertyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fibk_RealEstatePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fibk_AgriculturePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_AllOtherSegmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fibk_OtherPortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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

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

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

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

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

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

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

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

X
- Details
Name: | us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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