Loans Held for Investment - Schedule of Loans by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Deferred loan fees and costs	$ (20.5)		$ (21.0)
Loans held for investment, net of deferred fees and costs	9,622.5		9,807.5
Allowance for credit losses	(135.1)	$ (135.5)	(144.3)	$ (145.5)	$ (146.1)	$ (73.0)
Net loans held for investment	9,487.4		9,663.2
Commercial real estate
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	3,883.2		3,743.2
Allowance for credit losses	(51.8)	(51.1)	(54.8)	(55.1)	(51.2)	(19.5)
Financing Receivable, Loan in Process	10.4		12.8
Commercial real estate | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	6.1		7.6
Commercial real estate | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	4.0		1.2
Commercial real estate | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.3		4.0
Commercial real estate | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	3,864.0		3,720.8
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	260.2		265.0
Allowance for credit losses	(0.9)	(1.0)	(1.3)	(1.2)	(1.5)	(1.9)
Financing Receivable, Loan in Process	0.9		3.7
Land acquisition & development | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.7		2.5
Land acquisition & development | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		1.1
Land acquisition & development | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.2		0.1
Land acquisition & development | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	258.8		260.6
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	268.4		250.9
Allowance for credit losses	(1.8)	(1.5)	(1.6)	(1.6)	(1.3)	(1.5)
Financing Receivable, Loan in Process	0.0		1.9
Residential | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		1.5
Residential | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.4
Residential | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Residential | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	268.4		247.9
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	610.2		523.5
Allowance for credit losses	(6.7)	(8.0)	(7.3)	(7.1)	(6.3)	(2.7)
Financing Receivable, Loan in Process	1.9		12.2
Commercial | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.9		12.2
Commercial | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Commercial | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Commercial | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	608.3		511.2
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,138.8		1,039.4
Financing Receivable, Loan in Process	2.8		17.8
Total construction loans | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.6		16.2
Total construction loans | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		1.5
Total construction loans | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.2		0.1
Total construction loans | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1,135.5		1,019.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,554.9		1,396.3
Allowance for credit losses	(14.6)	(14.8)	(12.8)	(10.7)	(11.8)	(2.8)
Financing Receivable, Loan in Process	4.3		6.8
Residential | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.2		4.7
Residential | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.7		1.6
Residential | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.4		0.5
Residential | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1,546.3		1,384.9
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	229.9		220.6
Allowance for credit losses	(3.1)	(3.0)	(2.7)	(2.9)	(3.1)	(0.5)
Financing Receivable, Loan in Process	1.4		2.0
Agricultural | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		2.0
Agricultural | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.2		0.0
Agricultural | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.2		0.0
Agricultural | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	222.9		212.4
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	6,806.8		6,399.5
Allowance for credit losses	(78.9)	(79.4)	(80.5)	(78.6)	(75.2)	(28.9)
Financing Receivable, Loan in Process	18.9		39.4
Total real estate loans | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	8.9		30.5
Total real estate loans | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	6.9		4.3
Total real estate loans | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	3.1		4.6
Total real estate loans | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	6,768.7		6,337.8
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	756.8		805.1
Allowance for credit losses	(15.4)	(15.8)	(16.7)	(17.0)	(16.4)	(4.5)
Financing Receivable, Loan in Process	5.3		8.9
Indirect consumer | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	4.0		6.4
Indirect consumer | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.1		2.0
Indirect consumer | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.2		0.5
Indirect consumer | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	750.0		794.3
Direct and advance lines
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	132.9		150.6
Allowance for credit losses	(4.7)	(4.6)	(4.6)	(4.8)	(5.1)	(2.9)
Financing Receivable, Loan in Process	1.0		1.2
Direct and advance lines | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.7		0.8
Direct and advance lines | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.2		0.2
Direct and advance lines | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.1		0.2
Direct and advance lines | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	131.7		149.0
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	64.1		70.2
Allowance for credit losses	(1.8)	(1.6)	(2.6)	(2.2)	(2.0)	(2.5)
Financing Receivable, Loan in Process	1.2		1.6
Credit card | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.6		0.6
Credit card | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.2		0.4
Credit card | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.4		0.6
Credit card | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	62.9		68.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	953.8		1,025.9
Allowance for credit losses	(21.9)	(22.0)	(23.9)	(24.0)	(23.5)	(9.9)
Financing Receivable, Loan in Process	7.5		11.7
Total consumer loans | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	5.3		7.8
Total consumer loans | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.5		2.6
Total consumer loans | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.7		1.3
Total consumer loans | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	944.6		1,011.9
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,668.7		2,153.9
Allowance for credit losses	(33.9)	(33.7)	(39.2)	(42.0)	(46.5)	(32.6)
Financing Receivable, Loan in Process	5.2		9.2
Commercial | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	3.7		6.2
Commercial | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.6		1.8
Commercial | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.9		1.2
Commercial | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1,656.4		2,132.9
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	212.4		247.6
Allowance for credit losses	(0.4)	$ (0.4)	(0.7)	$ (0.9)	$ (0.9)	$ (1.6)
Financing Receivable, Loan in Process	0.8		2.4
Agricultural | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.3		0.4
Agricultural | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.6
Agricultural | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.5		1.4
Agricultural | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	209.7		242.1
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1.3		1.6
Financing Receivable, Loan in Process	0.0		0.0
Other, including overdrafts | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Other, including overdrafts | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Other, including overdrafts | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Other, including overdrafts | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.3		1.6
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	9,643.0		9,828.5
Financing Receivable, Loan in Process	32.4		62.7
Loans And Leases Held For Investment [Member] | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	18.2		44.9
Loans And Leases Held For Investment [Member] | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	9.0		9.3
Loans And Leases Held For Investment [Member] | Equal to or Greater than 90 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	5.2		8.5
Loans And Leases Held For Investment [Member] | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	$ 9,580.7		$ 9,726.3