Loans - Schedule of Loans by Segment (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment			$ 8,936.2
Deferred loan fees and costs	$ (41.3)		(5.5)
Loans held for investment, net of deferred fees and costs	10,032.5		8,930.7
Allowance for credit losses	(146.1)	$ (129.1)	(73.0)	$ (74.2)	$ (72.4)	$ (73.0)
Net loans held for investment	9,886.4		8,857.7
Commercial real estate
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	3,593.8		3,487.8
Allowance for credit losses	(51.2)	(39.5)	(19.5)
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	285.3		302.1
Allowance for credit losses	(1.5)	(1.4)	(1.9)
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	246.2		244.1
Allowance for credit losses	(1.3)	(1.1)	(1.5)
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	459.8		431.5
Allowance for credit losses	(6.3)	(6.1)	(2.7)
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	991.3		977.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,287.6		1,246.1
Allowance for credit losses	(11.8)	(14.3)	(2.8)
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	224.2		226.6
Allowance for credit losses	(3.1)	(2.6)	(0.5)
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	6,096.9		5,938.2
Allowance for credit losses	(75.2)	(65.0)	(28.9)
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	801.9		784.6
Allowance for credit losses	(16.4)	(16.7)	(4.5)
Direct and advance lines
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	169.3		179.0
Allowance for credit losses	(5.1)	(5.5)	(2.9)
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	70.6		81.6
Allowance for credit losses	(2.0)	(2.6)	(2.5)
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,041.8		1,045.2
Allowance for credit losses	(23.5)	(24.8)	(9.9)
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	2,648.6		1,673.7
Allowance for credit losses	(46.5)	(38.2)	(32.6)
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	282.8		279.1
Allowance for credit losses	(0.9)	$ (1.1)	(1.6)
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	3.7		0.0
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	$ 10,073.8		$ 8,936.2