Loans Held for Investment - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Impaired financing receivable, recorded investment	$ 94,400,000
Commitments to purchase or sell	0	$ 0
Proceeds from sale of loans held-for-investment	0	$ 0
Non-owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	103,400,000
2019	489,600,000
2018	299,300,000
2017	195,700,000
2016	102,200,000
Prior	495,400,000
Revolving Loans Amortized Cost Basis	20,800,000
Total	1,706,400,000
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	3,718,700,000		$ 3,743,200,000
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	30,900,000
2019	96,000,000.0
2018	51,500,000
2017	21,800,000
2016	29,200,000
Prior	31,900,000
Revolving Loans Amortized Cost Basis	1,900,000
Total	263,200,000
Loans held for investment	263,200,000		265,000,000.0
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	27,900,000
2019	64,600,000
2018	58,200,000
2017	1,300,000
2016	3,400,000
Prior	100,000
Revolving Loans Amortized Cost Basis	86,600,000
Total	242,100,000
Loans held for investment	242,100,000		250,900,000
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	61,100,000
2019	257,600,000
2018	191,700,000
2017	44,700,000
2016	11,000,000.0
Prior	5,800,000
Revolving Loans Amortized Cost Basis	9,100,000
Total	581,000,000.0
Loans held for investment	581,000,000.0		523,500,000
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,488,800,000		1,396,300,000
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,086,300,000		1,039,400,000
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	15,000,000.0
2019	49,700,000
2018	54,200,000
2017	30,800,000
2016	18,800,000
Prior	44,800,000
Revolving Loans Amortized Cost Basis	5,500,000
Total	218,800,000
Loans held for investment	218,800,000		220,600,000
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	6,512,600,000		6,399,500,000
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	70,500,000
2019	305,100,000
2018	166,400,000
2017	102,300,000
2016	62,400,000
Prior	76,200,000
Revolving Loans Amortized Cost Basis	0
Total	782,900,000
Loans held for investment	782,900,000		805,100,000
Direct consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	9,800,000
2019	41,400,000
2018	25,400,000
2017	23,800,000
2016	10,000,000.0
Prior	11,800,000
Revolving Loans Amortized Cost Basis	17,500,000
Total	139,700,000
Loans held for investment	139,700,000		150,600,000
Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	133.6
Loans held for investment	64,600,000		70,200,000
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	987,200,000		1,025,900,000
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,181,100,000		2,153,900,000
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	214,700,000		$ 247,600,000
Owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	71,800,000
2019	415,100,000
2018	308,800,000
2017	214,100,000
2016	127,300,000
Prior	511,100,000
Revolving Loans Amortized Cost Basis	7,200,000
Total	1,655,400,000
Multi-family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	19,800,000
2019	120,800,000
2018	54,600,000
2017	22,500,000
2016	39,400,000
Prior	99,000,000.0
Revolving Loans Amortized Cost Basis	800,000
Total	356,900,000
Commercial and floor plans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	882,100,000
2019	254,000,000.0
2018	140,000,000.0
2017	127,400,000
2016	71,400,000
Prior	135,300,000
Revolving Loans Amortized Cost Basis	240,700,000
Total	1,850,900,000
Commercial purpose secured by 1-4 family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	15,700,000
2019	78,300,000
2018	52,000,000.0
2017	33,100,000
2016	19,500,000
Prior	47,700,000
Revolving Loans Amortized Cost Basis	16,600,000
Total	262,900,000
Agriculture Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	21,000,000.0
2019	37,900,000
2018	15,600,000
2017	12,400,000
2016	4,300,000
Prior	3,800,000
Revolving Loans Amortized Cost Basis	118,100,000
Total	213,100,000
Residential 1-4 family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	151,100,000
2019	504,600,000
2018	94,300,000
2017	48,800,000
2016	39,700,000
Prior	269,900,000
Revolving Loans Amortized Cost Basis	1,500,000
Total	1,109,900,000
Consumer Home Equity and HELOC
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	3,000,000.0
2019	10,700,000
2018	6,500,000
2017	6,400,000
2016	8,700,000
Prior	17,900,000
Revolving Loans Amortized Cost Basis	325,700,000
Total	378,900,000
Pass | Non-owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	99,400,000
2019	472,000,000.0
2018	294,700,000
2017	194,000,000.0
2016	101,000,000.0
Prior	461,300,000
Revolving Loans Amortized Cost Basis	20,800,000
Total	1,643,200,000
Pass | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	30,300,000
2019	95,600,000
2018	51,400,000
2017	20,900,000
2016	28,600,000
Prior	30,500,000
Revolving Loans Amortized Cost Basis	1,300,000
Total	258,600,000
Pass | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	27,900,000
2019	64,600,000
2018	58,200,000
2017	1,100,000
2016	3,400,000
Prior	100,000
Revolving Loans Amortized Cost Basis	86,600,000
Total	241,900,000
Pass | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	61,100,000
2019	257,600,000
2018	191,700,000
2017	43,200,000
2016	11,000,000.0
Prior	5,800,000
Revolving Loans Amortized Cost Basis	9,100,000
Total	579,500,000
Pass | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	14,400,000
2019	45,800,000
2018	42,300,000
2017	26,400,000
2016	16,400,000
Prior	36,200,000
Revolving Loans Amortized Cost Basis	4,400,000
Total	185,900,000
Pass | Owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	70,400,000
2019	402,700,000
2018	300,300,000
2017	196,700,000
2016	119,300,000
Prior	454,700,000
Revolving Loans Amortized Cost Basis	7,000,000.0
Total	1,551,100,000
Pass | Multi-family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	19,800,000
2019	120,800,000
2018	54,600,000
2017	22,500,000
2016	39,400,000
Prior	98,900,000
Revolving Loans Amortized Cost Basis	800,000
Total	356,800,000
Pass | Commercial and floor plans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	880,100,000
2019	231,300,000
2018	137,000,000.0
2017	122,700,000
2016	64,300,000
Prior	125,600,000
Revolving Loans Amortized Cost Basis	231,100,000
Total	1,792,100,000
Pass | Commercial purpose secured by 1-4 family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	15,300,000
2019	75,600,000
2018	50,700,000
2017	30,500,000
2016	19,100,000
Prior	44,700,000
Revolving Loans Amortized Cost Basis	16,100,000
Total	252,000,000.0
Pass | Agriculture Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	20,100,000
2019	36,200,000
2018	13,700,000
2017	9,400,000
2016	3,700,000
Prior	3,100,000
Revolving Loans Amortized Cost Basis	102,900,000
Total	189,100,000
Other Assets Especially Mentioned | Non-owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	1,900,000
2018	1,900,000
2017	700,000
2016	0
Prior	19,600,000
Revolving Loans Amortized Cost Basis	0
Total	24,100,000
Other Assets Especially Mentioned | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	200,000
2018	100,000
2017	0
2016	400,000
Prior	1,100,000
Revolving Loans Amortized Cost Basis	500,000
Total	2,300,000
Other Assets Especially Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	0
2018	0
2017	1,500,000
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	1,500,000
Other Assets Especially Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	600,000
2019	2,500,000
2018	6,700,000
2017	1,100,000
2016	1,500,000
Prior	2,000,000.0
Revolving Loans Amortized Cost Basis	1,000,000.0
Total	15,400,000
Other Assets Especially Mentioned | Owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	500,000
2019	7,100,000
2018	1,700,000
2017	4,000,000.0
2016	4,200,000
Prior	35,600,000
Revolving Loans Amortized Cost Basis	0
Total	53,100,000
Other Assets Especially Mentioned | Commercial and floor plans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	500,000
2019	16,000,000.0
2018	1,300,000
2017	3,100,000
2016	6,500,000
Prior	4,000,000.0
Revolving Loans Amortized Cost Basis	4,300,000
Total	35,700,000
Other Assets Especially Mentioned | Commercial purpose secured by 1-4 family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	200,000
2018	500,000
2017	1,000,000.0
2016	100,000
Prior	1,100,000
Revolving Loans Amortized Cost Basis	400,000
Total	3,300,000
Other Assets Especially Mentioned | Agriculture Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	600,000
2019	1,000,000.0
2018	600,000
2017	300,000
2016	0
Prior	300,000
Revolving Loans Amortized Cost Basis	11,400,000
Total	14,200,000
Substandard | Non-owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	4,000,000.0
2019	15,700,000
2018	2,700,000
2017	1,000,000.0
2016	1,200,000
Prior	14,500,000
Revolving Loans Amortized Cost Basis	0
Total	39,100,000
Substandard | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	600,000
2019	200,000
2018	0
2017	900,000
2016	200,000
Prior	200,000
Revolving Loans Amortized Cost Basis	100,000
Total	2,200,000
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	0
2018	0
2017	200,000
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	200,000
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	1,400,000
2018	5,200,000
2017	3,300,000
2016	900,000
Prior	6,600,000
Revolving Loans Amortized Cost Basis	100,000
Total	17,500,000
Substandard | Owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	900,000
2019	5,100,000
2018	6,800,000
2017	13,400,000
2016	3,600,000
Prior	20,800,000
Revolving Loans Amortized Cost Basis	200,000
Total	50,800,000
Substandard | Multi-family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	100,000
Revolving Loans Amortized Cost Basis	0
Total	100,000
Substandard | Commercial and floor plans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	1,500,000
2019	6,400,000
2018	1,400,000
2017	1,600,000
2016	600,000
Prior	5,300,000
Revolving Loans Amortized Cost Basis	5,100,000
Total	21,900,000
Substandard | Commercial purpose secured by 1-4 family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	400,000
2019	2,500,000
2018	800,000
2017	1,600,000
2016	300,000
Prior	1,900,000
Revolving Loans Amortized Cost Basis	100,000
Total	7,600,000
Substandard | Agriculture Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	300,000
2019	700,000
2018	1,300,000
2017	2,700,000
2016	600,000
Prior	400,000
Revolving Loans Amortized Cost Basis	3,800,000
Total	9,800,000
Doubtful | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	100,000
Revolving Loans Amortized Cost Basis	0
Total	100,000
Doubtful | Owner occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	200,000
2018	0
2017	0
2016	200,000
Prior	0
Revolving Loans Amortized Cost Basis	0
Total	400,000
Doubtful | Commercial and floor plans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	300,000
2018	300,000
2017	0
2016	0
Prior	400,000
Revolving Loans Amortized Cost Basis	200,000
Total	1,200,000
Performing | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	70,500,000
2019	304,800,000
2018	165,800,000
2017	101,900,000
2016	62,200,000
Prior	75,600,000
Revolving Loans Amortized Cost Basis	0
Total	780,800,000
Performing | Direct consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	9,800,000
2019	41,400,000
2018	25,300,000
2017	23,600,000
2016	10,000,000.0
Prior	11,700,000
Revolving Loans Amortized Cost Basis	17,400,000
Total	139,200,000
Performing | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	132.8
Performing | Residential 1-4 family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	151,100,000
2019	504,400,000
2018	93,600,000
2017	48,800,000
2016	39,500,000
Prior	268,300,000
Revolving Loans Amortized Cost Basis	1,500,000
Total	1,107,200,000
Performing | Consumer Home Equity and HELOC
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	3,000,000.0
2019	10,700,000
2018	6,100,000
2017	6,400,000
2016	8,100,000
Prior	17,000,000.0
Revolving Loans Amortized Cost Basis	325,600,000
Total	376,900,000
Nonperforming | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	300,000
2018	600,000
2017	400,000
2016	200,000
Prior	600,000
Revolving Loans Amortized Cost Basis	0
Total	2,100,000
Nonperforming | Direct consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	0
2018	100,000
2017	200,000
2016	0
Prior	100,000
Revolving Loans Amortized Cost Basis	100,000
Total	500,000
Nonperforming | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0.8
Nonperforming | Residential 1-4 family
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	200,000
2018	700,000
2017	0
2016	200,000
Prior	1,600,000
Revolving Loans Amortized Cost Basis	0
Total	2,700,000
Nonperforming | Consumer Home Equity and HELOC
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0
2019	0
2018	400,000
2017	0
2016	600,000
Prior	900,000
Revolving Loans Amortized Cost Basis	100,000
Total	2,000,000.0
Agriculture Portfolio Segment [Member] | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	1.6
Agriculture Portfolio Segment [Member] | Performing | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	1.5
Agriculture Portfolio Segment [Member] | Nonperforming | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0.1
Commercial | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	67.4
Commercial | Performing | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	67.1
Commercial | Nonperforming | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	0.3
Total consumer loans | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	64.6
Total consumer loans | Performing | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	64.2
Total consumer loans | Nonperforming | Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
2020	$ 0.4