Loans - Schedule of Loans by Segment (Details) - USD ($)	Mar. 31, 2020	Mar. 27, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Net deferred loan fees and costs	$ (6,400,000)			$ (5,500,000)
Loans held for investment	8,918,000,000.0			8,930,700,000
Mortgage loans held for sale	94,400,000			100,900,000
Total loans, net of deferred fees and costs	9,012,400,000	$ 1,000,000,000.0		9,031,600,000
Allowance for credit losses	(129,100,000)		$ (29,000,000.0)	(73,000,000.0)	$ (72,400,000)	$ (73,000,000.0)
Net loans	8,883,300,000			8,958,600,000
Commercial real estate
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	3,543,400,000			3,487,800,000
Allowance for credit losses	(39,500,000)			(19,500,000)
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	290,500,000			302,100,000
Total loans, net of deferred fees and costs	290.5
Allowance for credit losses	(1,400,000)			(1,900,000)
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	228,400,000			244,100,000
Total loans, net of deferred fees and costs	228.4
Allowance for credit losses	(1,100,000)			(1,500,000)
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	459,400,000			431,500,000
Total loans, net of deferred fees and costs	459.4
Allowance for credit losses	(6,100,000)			(2,700,000)
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	978,300,000			977,700,000
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,231,000,000.0			1,246,100,000
Allowance for credit losses	(14,300,000)			(2,800,000)
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	224,600,000			226,600,000
Total loans, net of deferred fees and costs	224.6
Allowance for credit losses	(2,600,000)			(500,000)
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	5,977,300,000			5,938,200,000
Allowance for credit losses	(65,000,000.0)			(28,900,000)
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	774,600,000			784,600,000
Total loans, net of deferred fees and costs	774.6
Allowance for credit losses	(16,700,000)			(4,500,000)
Direct and advance lines
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	175,000,000.0			179,000,000.0
Total loans, net of deferred fees and costs	175.0
Allowance for credit losses	(5,500,000)			(2,900,000)
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	75,900,000			81,600,000
Allowance for credit losses	(2,600,000)			(2,500,000)
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,025,500,000			1,045,200,000
Allowance for credit losses	(24,800,000)			(9,900,000)
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,660,000,000.0			1,673,700,000
Allowance for credit losses	(38,200,000)			(32,600,000)
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	257,900,000			279,100,000
Allowance for credit losses	(1,100,000)			(1,600,000)
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	3,700,000			0
Mortgage loans originated for sale
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Mortgage loans held for sale	94,400,000			100,900,000
Total loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Total loans, net of deferred fees and costs	$ 9,012,400,000			$ 9,031,600,000