Gross carrying amount of all categories of loans and leases held in the portfolio, excluding loans held for sale.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of allowance for credit loss on accounts receivable.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the provision for loan losses.

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The effect of a change in method or methods for calculating the allowance for credit losses on the current period provision.

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Amount of recovery of financing receivables doubtful of collection that were previously charged off.

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Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts for loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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Amount, after allowance for bad debt, of loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

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Loans - Schedule of Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Dec. 31, 2017
Loans and Leases Receivable
Loans and Leases Receivable, Deferred Income	$ (21.0)	$ (5.5)
Mortgage loans held for sale, at fair value	74.0	100.9
Total loans	9,807.5	8,930.7
Accounts Receivable, Allowance for Credit Loss	(144.3)	(73.0)	$ (73.0)	$ (73.0)	$ (72.1)
Loans and Leases Receivable, Net Amount	9,663.2	8,857.7
Loans, principally due to allowance for credit losses	(36.5)	(18.1)
Less loans charged-off	(20.9)	(23.6)	$ (19.7)
Financing Receivable, Allowance for Credit Loss, Recovery	6.7
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	55.5
Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(30.0)
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable
Loans held for investment	3,743.2	3,487.8
Accounts Receivable, Allowance for Credit Loss	(54.8)			(19.5)
Loans, principally due to allowance for credit losses	(20.1)
Less loans charged-off	(0.4)
Financing Receivable, Allowance for Credit Loss, Recovery	0.3
Commercial Real Estate Portfolio Segment [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(15.3)
Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	250.9	244.1
Total loans	250.9
Accounts Receivable, Allowance for Credit Loss	(1.6)			(1.5)
Loans, principally due to allowance for credit losses	(1.0)
Less loans charged-off	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0
Residential Real Estate Construction Financing Receivable [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	0.9
Commercial Real Estate Construction [Member]
Loans and Leases Receivable
Loans held for investment	523.5	431.5
Total loans	523.5
Accounts Receivable, Allowance for Credit Loss	(7.3)			(2.7)
Loans, principally due to allowance for credit losses	(3.3)
Less loans charged-off	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0
Commercial Real Estate Construction [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(1.3)
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	1,039.4	977.7
Residential Mortgage [Member]
Loans and Leases Receivable
Loans held for investment	1,396.3	1,246.1
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	220.6	226.6
Total loans	220.6
Accounts Receivable, Allowance for Credit Loss	(2.7)			(0.5)
Loans, principally due to allowance for credit losses	(0.4)
Less loans charged-off	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0
Agricultural Real Estate Financing Receivable [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(1.8)
Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	6,399.5	5,938.2
Accounts Receivable, Allowance for Credit Loss	(80.5)			(28.9)
Loans, principally due to allowance for credit losses	(22.9)
Less loans charged-off	(0.9)
Financing Receivable, Allowance for Credit Loss, Recovery	1.1
Real Estate Financing Receivable [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(28.5)
Indirect consumer
Loans and Leases Receivable
Loans held for investment	805.1	784.6
Total loans	805.1
Accounts Receivable, Allowance for Credit Loss	(16.7)			(4.5)
Loans, principally due to allowance for credit losses	(5.4)
Less loans charged-off	(4.1)
Financing Receivable, Allowance for Credit Loss, Recovery	2.1
Indirect consumer \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(8.8)
Other consumer
Loans and Leases Receivable
Loans held for investment	150.6	179.0
Total loans	150.6
Accounts Receivable, Allowance for Credit Loss	(4.6)			(2.9)
Loans, principally due to allowance for credit losses	(1.6)
Less loans charged-off	(3.9)
Financing Receivable, Allowance for Credit Loss, Recovery	1.0
Other consumer \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(3.0)
Credit card consumer
Loans and Leases Receivable
Loans held for investment	70.2	81.6
Accounts Receivable, Allowance for Credit Loss	(2.6)			(2.5)
Loans, principally due to allowance for credit losses	(1.8)
Less loans charged-off	(2.8)
Financing Receivable, Allowance for Credit Loss, Recovery	0.8
Credit card consumer \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.3)
Total consumer loans
Loans and Leases Receivable
Loans held for investment	1,025.9	1,045.2
Accounts Receivable, Allowance for Credit Loss	(23.9)			(9.9)
Loans, principally due to allowance for credit losses	(8.8)
Less loans charged-off	(10.8)
Financing Receivable, Allowance for Credit Loss, Recovery	3.9
Total consumer loans \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(12.1)
Commercial Portfolio Segment [Member]
Loans and Leases Receivable
Loans held for investment	2,153.9	1,673.7
Accounts Receivable, Allowance for Credit Loss	(39.2)			(32.6)
Loans, principally due to allowance for credit losses	(24.0)
Less loans charged-off	(9.1)
Financing Receivable, Allowance for Credit Loss, Recovery	1.7
Commercial Portfolio Segment [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	10.0
Agricultural Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	247.6	279.1
Accounts Receivable, Allowance for Credit Loss	(0.7)			(1.6)
Loans, principally due to allowance for credit losses	(0.2)
Less loans charged-off	(0.1)
Financing Receivable, Allowance for Credit Loss, Recovery	0.0
Agricultural Financing Receivable [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	0.6
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	1.6	0.0
Loans held for investment
Loans and Leases Receivable
Loans held for investment	9,828.5	8,936.2
FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable
Total loans	1,674.5
Accounts Receivable, Allowance for Credit Loss	(25.5)			(8.8)
Loans, principally due to allowance for credit losses	(11.7)
Less loans charged-off	0.0
FIBK Commercial Real Estate Non Owner Occupied Loans \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(4.9)
Financing Receivable, Allowance for Credit Loss, Recovery	0.1
Commercial Real Estate Owner Occupied
Loans and Leases Receivable
Total loans	1,705.0
Accounts Receivable, Allowance for Credit Loss	(18.3)			(10.0)
Loans, principally due to allowance for credit losses	(5.0)
Less loans charged-off	(0.4)
Financing Receivable, Allowance for Credit Loss, Recovery	0.2
Commercial Real Estate Owner Occupied \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(3.5)
Commercial Real Estate Multifamily
Loans and Leases Receivable
Total loans	363.7
Accounts Receivable, Allowance for Credit Loss	(11.0)			(0.7)
Loans, principally due to allowance for credit losses	(3.4)
Less loans charged-off	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0
Commercial Real Estate Multifamily \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(6.9)
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	265.0	$ 302.1
Total loans	265.0
Accounts Receivable, Allowance for Credit Loss	(1.3)			(1.9)
Loans, principally due to allowance for credit losses	(0.4)
Less loans charged-off	(0.5)
Financing Receivable, Allowance for Credit Loss, Recovery	0.4
Land Acquisition And Development Construction Financing Receivable [Member] \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	0.1
Construction Loans
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(10.2)			(6.1)
Loans, principally due to allowance for credit losses	(3.9)
Less loans charged-off	(0.5)
Financing Receivable, Allowance for Credit Loss, Recovery	0.4
Construction Loans \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.3)
Residential Real Estate 1-4 Family
Loans and Leases Receivable
Total loans	1,012.3
Accounts Receivable, Allowance for Credit Loss	(11.4)			(1.8)
Loans, principally due to allowance for credit losses	(1.1)
Less loans charged-off	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.1
Residential Real Estate 1-4 Family \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(10.6)
Home Equity Loan
Loans and Leases Receivable
Total loans	384.0
Accounts Receivable, Allowance for Credit Loss	(1.4)			(1.0)
Loans, principally due to allowance for credit losses	(0.4)
Less loans charged-off	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.3
Home Equity Loan \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.5)
Residential Real Estate
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(12.8)			(2.8)
Loans, principally due to allowance for credit losses	(1.5)
Less loans charged-off	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.4
Residential Real Estate \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(11.1)
Commercial and Floor Plans
Loans and Leases Receivable
Total loans	1,814.5
Accounts Receivable, Allowance for Credit Loss	(34.2)			(25.5)
Loans, principally due to allowance for credit losses	(20.4)
Less loans charged-off	(8.0)
Financing Receivable, Allowance for Credit Loss, Recovery	1.4
Commercial and Floor Plans \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	5.1
Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable
Total loans	272.8
Accounts Receivable, Allowance for Credit Loss	(4.7)			(5.9)
Loans, principally due to allowance for credit losses	(2.5)
Less loans charged-off	(0.1)
Financing Receivable, Allowance for Credit Loss, Recovery	0.2
Commercial Purpose Secured by 1-4 Family \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	3.8
Commercial Borrower
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.3)			$ (1.2)
Loans, principally due to allowance for credit losses	(1.1)
Less loans charged-off	(1.0)
Financing Receivable, Allowance for Credit Loss, Recovery	0.1
Commercial Borrower \| Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	$ 1.1