Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Allowance	$ 9,331,700,000	$ 9,807,500,000
Loans and Leases Receivable, Commitments to Purchase or Sell	0
Proceeds from Sale of Loans and Leases Held-for-investment	0
FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	512,000,000.0	511,900,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	470,600,000	309,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	242,300,000	218,100,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	151,100,000	109,500,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	77,300,000	147,200,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(425,000,000.0)	(364,100,000)
Financing Receivable, Revolving	15,300,000	13,800,000
Loans and Leases Receivable, Allowance	1,893,600,000	1,674,500,000
Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	457,800,000	432,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	322,400,000	328,600,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	241,200,000	227,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	163,800,000	130,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,700,000	187,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(356,600,000)	(389,100,000)
Financing Receivable, Revolving	14,200,000	9,200,000
Loans and Leases Receivable, Allowance	1,656,700,000	1,705,000,000
Commercial Real Estate Multifamily
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	129,100,000	132,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	118,600,000	58,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	43,900,000	23,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	15,400,000	41,600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	36,000,000.0	25,800,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(76,700,000)	(80,600,000)
Financing Receivable, Revolving	1,500,000	800,000
Loans and Leases Receivable, Allowance	421,200,000	363,700,000
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	247,800,000	265,000,000.0
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]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	262,000,000.0	250,900,000
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]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	498,000,000.0	523,500,000
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
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	213,900,000	220,600,000
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
Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	396,300,000	1,044,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	179,900,000	156,200,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	96,900,000	142,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	76,900,000	77,200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	50,700,000	51,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(97,700,000)	(96,500,000)
Financing Receivable, Revolving	234,300,000	247,200,000
Loans and Leases Receivable, Allowance	1,132,700,000	1,814,500,000
Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	96,200,000	85,200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	56,400,000	55,200,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	28,600,000	38,800,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	24,200,000	21,100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	15,600,000	17,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(34,100,000)	(36,700,000)
Financing Receivable, Revolving	14,500,000	18,400,000
Loans and Leases Receivable, Allowance	269,600,000	272,800,000
Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	40,200,000	52,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	21,000,000.0	20,300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	9,700,000	15,300,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	8,300,000	4,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	2,700,000	3,200,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(2,000,000.0)	(2,000,000.0)
Financing Receivable, Revolving	118,500,000	147,900,000
Loans and Leases Receivable, Allowance	202,400,000	246,100,000
Residential Real Estate 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	360,900,000	491,200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	477,300,000	114,600,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	74,700,000	57,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	27,500,000	47,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	25,900,000	65,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(177,300,000)	(235,800,000)
Financing Receivable, Revolving	0	0
Loans and Leases Receivable, Allowance	1,143,600,000	1,012,300,000
Residential Real Estate 1-4 Family | Performing Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	360,900,000	491,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	477,000,000.0	113,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	74,700,000	57,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	27,500,000	47,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	25,700,000	65,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(176,500,000)	(234,600,000)
Financing Receivable, Revolving	0	0
Loans and Leases Receivable, Allowance	1,142,300,000	1,010,300,000
Residential Real Estate 1-4 Family | Nonperforming Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0	100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	300,000	700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	0	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	200,000	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(800,000)	(1,200,000)
Financing Receivable, Revolving	0	0
Loans and Leases Receivable, Allowance	1,300,000	2,000,000.0
Home Equity Loan
Loans and Leases Receivable Recorded Investment, Criticized Loans
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	737,600,000	805,100,000
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	129,200,000	150,600,000
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
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
Loans and Leases Receivable Recorded Investment, Criticized Loans
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
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	931,700,000	1,025,900,000
Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	64,900,000	70,200,000
Financing Receivable, Originated in Current Fiscal Year	139,600,000	138.3
Credit card consumer | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	64,900,000	70.2
Credit card consumer | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	73,200,000	66.6
Credit card consumer | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,500,000	1.5
Credit card consumer | Performing Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	139,000,000.0	137.4
Credit card consumer | Performing Financial Instruments | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	64,400,000	69.6
Credit card consumer | Performing Financial Instruments | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	73,100,000	66.3
Credit card consumer | Performing Financial Instruments | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,500,000	1.5
Credit card consumer | Nonperforming Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	600,000	0.9
Credit card consumer | Nonperforming Financial Instruments | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	500,000	0.6
Credit card consumer | Nonperforming Financial Instruments | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	100,000	0.3
Credit card consumer | Nonperforming Financial Instruments | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0	0
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,475,500,000	2,153,900,000
Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	203,900,000	247,600,000
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,971,500,000	3,743,200,000
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,007,800,000	1,039,400,000
Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,538,200,000	1,396,300,000
Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6,731,400,000	6,399,500,000
Pass | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	507,900,000	495,900,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	452,200,000	304,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	237,900,000	216,000,000.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	150,400,000	105,300,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	76,300,000	139,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(409,000,000.0)	(336,500,000)
Financing Receivable, Revolving	15,300,000	13,800,000
Loans and Leases Receivable, Allowance	1,849,000,000	1,612,100,000
Pass | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	452,700,000	416,300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	314,900,000	312,500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	235,000,000.0	211,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	151,000,000.0	122,400,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	94,500,000	153,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(322,500,000)	(357,900,000)
Financing Receivable, Revolving	14,200,000	8,900,000
Loans and Leases Receivable, Allowance	1,584,800,000	1,582,900,000
Pass | Commercial Real Estate Multifamily
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	129,100,000	132,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	118,600,000	58,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	43,900,000	23,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	15,400,000	41,600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	36,000,000.0	25,800,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(76,700,000)	(80,500,000)
Financing Receivable, Revolving	1,500,000	800,000
Loans and Leases Receivable, Allowance	421,200,000	363,600,000
Pass | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	113,000,000.0	104,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	41,500,000	58,800,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	34,200,000	26,400,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	14,800,000	30,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	19,800,000	7,600,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(20,800,000)	(26,300,000)
Financing Receivable, Revolving	1,200,000	5,800,000
Loans and Leases Receivable, Allowance	245,300,000	260,200,000
Pass | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	112,400,000	80,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	7,000,000.0	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	0	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	0	(100,000)
Financing Receivable, Revolving	127,200,000	82,100,000
Loans and Leases Receivable, Allowance	261,200,000	249,600,000
Pass | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
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	61,200,000
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)	(300,000)
Financing Receivable, Revolving	0	11,600,000
Loans and Leases Receivable, Allowance	498,000,000.0	521,900,000
Pass | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	58,300,000	50,000,000.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	36,900,000	45,300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	35,100,000	28,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	22,600,000	17,200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	11,800,000	12,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(28,100,000)	(27,200,000)
Financing Receivable, Revolving	4,900,000	5,300,000
Loans and Leases Receivable, Allowance	197,700,000	185,900,000
Pass | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	394,200,000	1,029,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	165,700,000	153,000,000.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	94,500,000	136,000,000.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	73,500,000	69,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	47,100,000	43,000,000.0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(91,300,000)	(92,300,000)
Financing Receivable, Revolving	224,700,000	233,100,000
Loans and Leases Receivable, Allowance	1,091,000,000	1,756,600,000
Pass | Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	94,900,000	82,300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	55,000,000.0	53,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	27,800,000	33,700,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	23,100,000	20,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	15,300,000	15,500,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(32,200,000)	(34,900,000)
Financing Receivable, Revolving	14,400,000	17,500,000
Loans and Leases Receivable, Allowance	262,700,000	258,300,000
Pass | Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	35,100,000	47,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	16,200,000	18,100,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	9,000,000.0	10,700,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	5,400,000	4,200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	2,100,000	3,000,000.0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,600,000)	(1,300,000)
Financing Receivable, Revolving	108,900,000	130,900,000
Loans and Leases Receivable, Allowance	178,300,000	215,600,000
Other Assets Especial Mentioned | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	200,000	300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	3,100,000	2,300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	2,100,000	900,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	0	100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	0	6,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(3,600,000)	(13,700,000)
Financing Receivable, Revolving	0	0
Loans and Leases Receivable, Allowance	9,000,000.0	23,700,000
Other Assets Especial Mentioned | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,300,000	7,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	3,200,000	9,600,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,500,000	4,800,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	7,400,000	3,100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	3,500,000	18,600,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(13,800,000)	(20,000,000.0)
Financing Receivable, Revolving	0	0
Loans and Leases Receivable, Allowance	30,700,000	63,200,000
Other Assets Especial Mentioned | Commercial Real Estate Multifamily
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		0
Financing Receivable, Originated Two Years before Latest Fiscal Year		0
Financing Receivable, Originated Three Years before Latest Fiscal Year		0
Financing Receivable, Originated Four Years before Latest Fiscal Year		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		(100,000)
Financing Receivable, Revolving		0
Loans and Leases Receivable, Allowance		100,000
Other Assets Especial Mentioned | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0	200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	100,000	100,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	0	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0	900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(300,000)	(1,200,000)
Financing Receivable, Revolving	0	500,000
Loans and Leases Receivable, Allowance	500,000	2,900,000
Other Assets Especial Mentioned | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		0
Financing Receivable, Originated Two Years before Latest Fiscal Year		1,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year		0
Financing Receivable, Originated Four Years before Latest Fiscal Year		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		0
Financing Receivable, Revolving		0
Loans and Leases Receivable, Allowance		1,500,000
Other Assets Especial Mentioned | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	100,000	2,800,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	1,300,000	6,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,200,000	1,000,000.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	100,000	1,500,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000	900,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(900,000)	(1,000,000.0)
Financing Receivable, Revolving	900,000	500,000
Loans and Leases Receivable, Allowance	4,600,000	14,400,000
Other Assets Especial Mentioned | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	800,000	5,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	11,400,000	1,000,000.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	800,000	1,900,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	800,000	7,000,000.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	3,000,000.0	3,900,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(2,300,000)	(500,000)
Financing Receivable, Revolving	7,000,000.0	2,400,000
Loans and Leases Receivable, Allowance	26,100,000	22,300,000
Other Assets Especial Mentioned | Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0	500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	200,000	500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	200,000	300,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	500,000	100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000	500,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(600,000)	(800,000)
Financing Receivable, Revolving	0	800,000
Loans and Leases Receivable, Allowance	1,600,000	3,500,000
Other Assets Especial Mentioned | Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	200,000	1,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	4,100,000	700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	100,000	400,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	400,000	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	(300,000)	(300,000)
Financing Receivable, Revolving	7,000,000.0	13,400,000
Loans and Leases Receivable, Allowance	12,700,000	16,400,000
Substandard | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	3,900,000	15,700,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	15,300,000	2,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	2,300,000	1,000,000.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	700,000	4,100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	1,000,000.0	1,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(12,400,000)	(13,900,000)
Financing Receivable, Revolving	0	0
Loans and Leases Receivable, Allowance	35,600,000	38,500,000
Substandard | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	3,800,000	8,900,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	4,300,000	6,500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	4,700,000	11,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	5,400,000	5,000,000.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	2,700,000	15,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(20,300,000)	(11,100,000)
Financing Receivable, Revolving	0	300,000
Loans and Leases Receivable, Allowance	41,200,000	58,400,000
Substandard | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	800,000	300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	200,000	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0	1,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	600,000	100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	300,000	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)	(100,000)
Financing Receivable, Revolving	0	100,000
Loans and Leases Receivable, Allowance	2,000,000.0	1,800,000
Substandard | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0	200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	400,000	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	400,000	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	0	0
Financing Receivable, Revolving	0	1,100,000
Loans and Leases Receivable, Allowance	800,000	1,300,000
Substandard | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		0
Financing Receivable, Originated Two Years before Latest Fiscal Year		0
Financing Receivable, Originated Three Years before Latest Fiscal Year		0
Financing Receivable, Originated Four Years before Latest Fiscal Year		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		(100,000)
Financing Receivable, Revolving		0
Loans and Leases Receivable, Allowance		100,000
Substandard | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	4,000,000.0	1,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	400,000	5,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,000,000.0	3,400,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	600,000	900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	1,300,000	3,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(4,300,000)	(4,400,000)
Financing Receivable, Revolving	0	600,000
Loans and Leases Receivable, Allowance	11,600,000	20,000,000.0
Substandard | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,300,000	8,800,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	2,800,000	1,800,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,600,000	4,300,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	2,600,000	400,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	600,000	4,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(4,100,000)	(1,100,000)
Financing Receivable, Revolving	2,600,000	11,500,000
Loans and Leases Receivable, Allowance	15,600,000	32,000,000.0
Substandard | Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,300,000	2,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	1,200,000	1,000,000.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	600,000	4,800,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	600,000	300,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	200,000	1,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,300,000)	(1,000,000.0)
Financing Receivable, Revolving	100,000	100,000
Loans and Leases Receivable, Allowance	5,300,000	11,000,000.0
Substandard | Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	4,900,000	3,700,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	700,000	1,500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	600,000	4,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	2,500,000	600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	0	100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)	(400,000)
Financing Receivable, Revolving	2,600,000	3,600,000
Loans and Leases Receivable, Allowance	$ 11,400,000	14,100,000
Doubtful | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		0
Financing Receivable, Originated Two Years before Latest Fiscal Year		200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year		0
Financing Receivable, Originated Four Years before Latest Fiscal Year		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		0
Financing Receivable, Revolving		0
Loans and Leases Receivable, Allowance		200,000
Doubtful | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		0
Financing Receivable, Originated Two Years before Latest Fiscal Year		0
Financing Receivable, Originated Three Years before Latest Fiscal Year		200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		(100,000)
Financing Receivable, Revolving		0
Loans and Leases Receivable, Allowance		500,000
Doubtful | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		0
Financing Receivable, Originated Two Years before Latest Fiscal Year		0
Financing Receivable, Originated Three Years before Latest Fiscal Year		0
Financing Receivable, Originated Four Years before Latest Fiscal Year		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		(100,000)
Financing Receivable, Revolving		0
Loans and Leases Receivable, Allowance		100,000
Doubtful | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year		0
Financing Receivable, Originated Three Years before Latest Fiscal Year		0
Financing Receivable, Originated Four Years before Latest Fiscal Year		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		0
Financing Receivable, Revolving		0
Loans and Leases Receivable, Allowance		300,000
Doubtful | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year		300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		400,000
Financing Receivable, Originated Two Years before Latest Fiscal Year		0
Financing Receivable, Originated Three Years before Latest Fiscal Year		0
Financing Receivable, Originated Four Years before Latest Fiscal Year		100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		(2,600,000)
Financing Receivable, Revolving		200,000
Loans and Leases Receivable, Allowance		$ 3,600,000