Loans - Schedule of Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021	Dec. 31, 2018
Loans and Leases Receivable
Loans and Leases Receivable, Deferred Income	$ (12.3)	$ (21.0)
Mortgage loans held for sale, at fair value	30.1	74.0
Total loans	9,331.7	9,807.5
Accounts Receivable, Allowance for Credit Loss	(122.3)	(144.3)	$ (144.3)	$ (73.0)
Loans and Leases Receivable, Net Amount	9,209.4	9,663.2
Less loans charged-off	14.7	(55.5)
Financing Receivable, Allowance for Credit Loss, Writeoff	16.6	20.9
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		30.0
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	144.3	73.0
Provision for Loan, Lease, and Other Losses	(14.7)	55.5
Financing Receivable, Allowance for Credit Loss, Recovery	9.3	6.7
Financing Receivable, Allowance for Credit Loss, Ending Balance	122.3	144.3
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable
Loans held for investment	3,971.5	3,743.2
Accounts Receivable, Allowance for Credit Loss			(54.8)
Less loans charged-off	8.7	(20.1)
Financing Receivable, Allowance for Credit Loss, Writeoff	2.3	0.4
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		15.3
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		19.5
Provision for Loan, Lease, and Other Losses	(8.7)	20.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.3
Financing Receivable, Allowance for Credit Loss, Ending Balance	43.9	54.8
Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	262.0	250.9
Total loans	262.0	250.9
Accounts Receivable, Allowance for Credit Loss			(1.6)
Less loans charged-off	(0.9)	(1.0)
Financing Receivable, Allowance for Credit Loss, Writeoff	0.1	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		0.9
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		1.5
Provision for Loan, Lease, and Other Losses	0.9	1.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	2.4	1.6
Commercial Real Estate Construction [Member]
Loans and Leases Receivable
Loans held for investment	498.0	523.5
Total loans	498.0	523.5
Accounts Receivable, Allowance for Credit Loss			(7.3)
Less loans charged-off	1.3	(3.3)
Financing Receivable, Allowance for Credit Loss, Writeoff	0.1	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		1.3
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		2.7
Provision for Loan, Lease, and Other Losses	(1.3)	3.3
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	6.0	7.3
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	1,007.8	1,039.4
Residential Mortgage [Member]
Loans and Leases Receivable
Loans held for investment	1,538.2	1,396.3
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	213.9	220.6
Total loans	213.9	220.6
Accounts Receivable, Allowance for Credit Loss			(2.7)
Less loans charged-off	0.1	(0.4)
Financing Receivable, Allowance for Credit Loss, Writeoff	0.7	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		1.8
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		0.5
Provision for Loan, Lease, and Other Losses	(0.1)	0.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	1.9	2.7
Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	6,731.4	6,399.5
Accounts Receivable, Allowance for Credit Loss			(80.5)
Less loans charged-off	7.7	(22.9)
Financing Receivable, Allowance for Credit Loss, Writeoff	4.5	0.9
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		28.5
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		28.9
Provision for Loan, Lease, and Other Losses	(7.7)	22.9
Financing Receivable, Allowance for Credit Loss, Recovery	1.0	1.1
Financing Receivable, Allowance for Credit Loss, Ending Balance	69.3	80.5
Indirect consumer
Loans and Leases Receivable
Loans held for investment	737.6	805.1
Total loans	737.6	805.1
Accounts Receivable, Allowance for Credit Loss			(16.7)
Less loans charged-off	1.4	(5.4)
Financing Receivable, Allowance for Credit Loss, Writeoff	3.5	4.1
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		8.8
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		4.5
Provision for Loan, Lease, and Other Losses	(1.4)	5.4
Financing Receivable, Allowance for Credit Loss, Recovery	2.5	2.1
Financing Receivable, Allowance for Credit Loss, Ending Balance	14.3	16.7
Other consumer
Loans and Leases Receivable
Loans held for investment	129.2	150.6
Total loans	129.2	150.6
Accounts Receivable, Allowance for Credit Loss			(4.6)
Less loans charged-off	(1.7)	(1.6)
Financing Receivable, Allowance for Credit Loss, Writeoff	2.9	3.9
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		3.0
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		2.9
Provision for Loan, Lease, and Other Losses	1.7	1.6
Financing Receivable, Allowance for Credit Loss, Recovery	1.2	1.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	4.6	4.6
Credit card consumer
Loans and Leases Receivable
Loans held for investment	64.9	70.2
Accounts Receivable, Allowance for Credit Loss			(2.6)
Less loans charged-off	(0.6)	(1.8)
Financing Receivable, Allowance for Credit Loss, Writeoff	1.8	2.8
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		0.3
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		2.5
Provision for Loan, Lease, and Other Losses	0.6	1.8
Financing Receivable, Allowance for Credit Loss, Recovery	0.8	0.8
Financing Receivable, Allowance for Credit Loss, Ending Balance	2.2	2.6
Total consumer loans
Loans and Leases Receivable
Loans held for investment	931.7	1,025.9
Accounts Receivable, Allowance for Credit Loss			(23.9)
Less loans charged-off	(0.9)	(8.8)
Financing Receivable, Allowance for Credit Loss, Writeoff	8.2	10.8
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		12.1
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		9.9
Provision for Loan, Lease, and Other Losses	0.9	8.8
Financing Receivable, Allowance for Credit Loss, Recovery	4.5	3.9
Financing Receivable, Allowance for Credit Loss, Ending Balance	21.1	23.9
Commercial Portfolio Segment [Member]
Loans and Leases Receivable
Loans held for investment	1,475.5	2,153.9
Accounts Receivable, Allowance for Credit Loss			(39.2)
Less loans charged-off	7.7	(24.0)
Financing Receivable, Allowance for Credit Loss, Writeoff	3.7	9.1
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		10.0
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		32.6
Provision for Loan, Lease, and Other Losses	(7.7)	24.0
Financing Receivable, Allowance for Credit Loss, Recovery	3.8	1.7
Financing Receivable, Allowance for Credit Loss, Ending Balance	31.6	39.2
Agricultural Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	203.9	247.6
Accounts Receivable, Allowance for Credit Loss			(0.7)
Less loans charged-off	0.2	0.2
Financing Receivable, Allowance for Credit Loss, Writeoff	0.2	0.1
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		0.6
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		1.6
Provision for Loan, Lease, and Other Losses	(0.2)	(0.2)
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	0.3	0.7
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	1.5	1.6
Loans held for investment
Loans and Leases Receivable
Loans held for investment	9,344.0	9,828.5
FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable
Total loans	1,893.6	1,674.5
Accounts Receivable, Allowance for Credit Loss			(25.5)
Less loans charged-off	8.3	(11.7)
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		4.9
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		8.8
Provision for Loan, Lease, and Other Losses	(8.3)	11.7
Financing Receivable, Allowance for Credit Loss, Recovery		0.1
Financing Receivable, Allowance for Credit Loss, Ending Balance	17.3	25.5
FIBK Commercial Real Estate Non Owner Occupied Loans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Financing Receivable, Allowance for Credit Loss, Writeoff		0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.1
Commercial Real Estate Owner Occupied
Loans and Leases Receivable
Total loans	1,656.7	1,705.0
Accounts Receivable, Allowance for Credit Loss			(18.3)
Less loans charged-off	2.7	(5.0)
Financing Receivable, Allowance for Credit Loss, Writeoff	2.3	0.4
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		3.5
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		10.0
Provision for Loan, Lease, and Other Losses	(2.7)	5.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.2
Financing Receivable, Allowance for Credit Loss, Ending Balance	13.3	18.3
Commercial Real Estate Multifamily
Loans and Leases Receivable
Total loans	421.2	363.7
Accounts Receivable, Allowance for Credit Loss			(11.0)
Less loans charged-off	(2.3)	(3.4)
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		6.9
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		0.7
Provision for Loan, Lease, and Other Losses	2.3	3.4
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	13.3	11.0
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	247.8	265.0
Total loans	247.8	265.0
Accounts Receivable, Allowance for Credit Loss			(1.3)
Less loans charged-off	0.1	0.4
Financing Receivable, Allowance for Credit Loss, Writeoff	1.2	0.5
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		0.1
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		1.9
Provision for Loan, Lease, and Other Losses	(0.1)	(0.4)
Financing Receivable, Allowance for Credit Loss, Recovery	0.5	0.4
Financing Receivable, Allowance for Credit Loss, Ending Balance	0.5	1.3
Construction Loans
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss			(10.2)
Less loans charged-off	0.5	(3.9)
Financing Receivable, Allowance for Credit Loss, Writeoff	1.4	0.5
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		0.3
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		6.1
Provision for Loan, Lease, and Other Losses	(0.5)	3.9
Financing Receivable, Allowance for Credit Loss, Recovery	0.6	0.4
Financing Receivable, Allowance for Credit Loss, Ending Balance	8.9	10.2
Residential Real Estate 1-4 Family
Loans and Leases Receivable
Total loans	1,143.6	1,012.3
Accounts Receivable, Allowance for Credit Loss			(11.4)
Less loans charged-off	(2.0)	1.1
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		(10.6)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		1.8
Provision for Loan, Lease, and Other Losses	2.0	(1.1)
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.1
Financing Receivable, Allowance for Credit Loss, Ending Balance	13.4	11.4
Home Equity Loan
Loans and Leases Receivable
Total loans	394.6	384.0
Accounts Receivable, Allowance for Credit Loss			(1.4)
Less loans charged-off	0.4	0.4
Financing Receivable, Allowance for Credit Loss, Writeoff	0.1	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		(0.5)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		1.0
Provision for Loan, Lease, and Other Losses	(0.4)	(0.4)
Financing Receivable, Allowance for Credit Loss, Recovery	0.3	0.3
Financing Receivable, Allowance for Credit Loss, Ending Balance	1.2	1.4
Residential Real Estate
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss			(12.8)
Less loans charged-off	(1.6)	1.5
Financing Receivable, Allowance for Credit Loss, Writeoff	0.1	0.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		(11.1)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		2.8
Provision for Loan, Lease, and Other Losses	1.6	(1.5)
Financing Receivable, Allowance for Credit Loss, Recovery	0.3	0.4
Financing Receivable, Allowance for Credit Loss, Ending Balance	14.6	12.8
Commercial and Floor Plans
Loans and Leases Receivable
Total loans	1,132.7	1,814.5
Accounts Receivable, Allowance for Credit Loss			(34.2)
Less loans charged-off	7.3	(20.4)
Financing Receivable, Allowance for Credit Loss, Writeoff	3.0	8.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		5.1
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		25.5
Provision for Loan, Lease, and Other Losses	(7.3)	20.4
Financing Receivable, Allowance for Credit Loss, Recovery	3.2	1.4
Financing Receivable, Allowance for Credit Loss, Ending Balance	27.1	34.2
Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable
Total loans	269.6	272.8
Accounts Receivable, Allowance for Credit Loss			(4.7)
Less loans charged-off	0.5	(2.5)
Financing Receivable, Allowance for Credit Loss, Writeoff	0.3	0.1
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		3.8
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		5.9
Provision for Loan, Lease, and Other Losses	(0.5)	2.5
Financing Receivable, Allowance for Credit Loss, Recovery	0.5	0.2
Financing Receivable, Allowance for Credit Loss, Ending Balance	4.4	4.7
Commercial Borrower
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss			$ (0.3)
Less loans charged-off	(0.1)	(1.1)
Financing Receivable, Allowance for Credit Loss, Writeoff	0.4	1.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		1.1
Accounts Receivable, Allowance for Credit Loss, Beginning Balance		1.2
Provision for Loan, Lease, and Other Losses	0.1	1.1
Financing Receivable, Allowance for Credit Loss, Recovery	0.1	0.1
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 0.1	$ 0.3