Loans - Schedule of Loans by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 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	$ (34.4)		(5.5)
Loans held for investment, net of deferred fees and costs	10,152.2		8,930.7
Allowance for credit losses	(145.5)	$ (146.1)	(73.0)	$ (75.0)	$ (74.2)	$ (73.0)
Net loans held for investment	10,006.7		8,857.7
Commercial real estate
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	3,690.9		3,487.8
Allowance for credit losses	(55.1)	(51.2)	(19.5)
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	274.8		302.1
Allowance for credit losses	(1.2)	(1.5)	(1.9)
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	227.9		244.1
Allowance for credit losses	(1.6)	(1.3)	(1.5)
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	530.8		431.5
Allowance for credit losses	(7.1)	(6.3)	(2.7)
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,033.5		977.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,311.2		1,246.1
Allowance for credit losses	(10.7)	(11.8)	(2.8)
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	227.7		226.6
Allowance for credit losses	(2.9)	(3.1)	(0.5)
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	6,263.3		5,938.2
Allowance for credit losses	(78.6)	(75.2)	(28.9)
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	812.8		784.6
Allowance for credit losses	(17.0)	(16.4)	(4.5)
Direct and advance lines
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	162.1		179.0
Allowance for credit losses	(4.8)	(5.1)	(2.9)
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	69.9		81.6
Allowance for credit losses	(2.2)	(2.0)	(2.5)
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,044.8		1,045.2
Allowance for credit losses	(24.0)	(23.5)	(9.9)
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	2,599.6		1,673.7
Allowance for credit losses	(42.0)	(46.5)	(32.6)
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	274.7		279.1
Allowance for credit losses	(0.9)	$ (0.9)	(1.6)
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	4.2		0.0
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	$ 10,186.6		$ 8,936.2