Loans Held for Investment - Schedule of Loans by Segment (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Deferred loan fees and costs	$ (13.1)		$ (15.6)
Loans held for investment, net of deferred fees and costs	18,213.3		18,099.2
Allowance for credit losses	(226.7)	$ (224.6)	(220.1)	$ (213.0)	$ (220.4)	$ (122.3)
Net loans held for investment	17,986.6		17,879.1
Nonaccrual loans	31.6		39.8
Commercial real estate
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(73.7)	(89.5)	(43.9)
Financing Receivable, Loan in Process	1.6		7.5
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	8,766.2		8,528.6
Nonaccrual loans	31.0		19.6
Commercial real estate | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.2		5.6
Commercial real estate | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.4		0.8
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	0.0		1.1
Commercial real estate | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	8,733.6		8,501.5
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(3.1)	(3.4)	(2.4)
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(24.7)	(14.9)	(6.0)
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	14.5		11.6
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1,930.3		1,944.4
Nonaccrual loans	17.4		3.7
Total construction loans | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	6.5		10.4
Total construction loans | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	7.5		0.6
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	0.5		0.6
Total construction loans | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1,898.4		1,929.1
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(20.7)	(19.7)	(14.6)
Financing Receivable, Loan in Process	5.2		13.2
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	2,212.2		2,188.3
Nonaccrual loans	9.5		6.4
Residential | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.4		9.9
Residential | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.8		2.1
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	1.0		1.2
Residential | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2,197.5		2,168.7
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(5.8)	(5.4)	(1.9)
Financing Receivable, Loan in Process	1.9		7.2
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	731.5		794.9
Nonaccrual loans	5.4		7.6
Agricultural | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.4		1.1
Agricultural | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.5		6.1
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	0.0		0.0
Agricultural | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	724.2		780.1
Total real estate
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses	(162.2)	(158.7)	(138.7)	(128.7)	(133.7)	(69.3)
Financing Receivable, Loan in Process	23.2		39.5
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	13,640.2		13,456.2
Nonaccrual loans	63.3		37.3
Total real estate | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	10.5		27.0
Total real estate | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	11.2		9.6
Total 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	1.5		2.9
Total real estate | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	13,553.7		13,379.4
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(14.1)	(13.1)	(14.3)
Financing Receivable, Loan in Process	9.0		12.3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	751.7		829.7
Nonaccrual loans	3.0		2.7
Indirect consumer | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	6.4		9.3
Indirect consumer | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.2		2.4
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	0.4		0.6
Indirect consumer | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	739.7		814.7
Direct and advance lines
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(5.3)	(5.0)	(4.6)
Financing Receivable, Loan in Process	0.8		1.2
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	142.3		152.9
Nonaccrual loans	0.2		0.3
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	0.6		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		0.3
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	0.0		0.1
Direct and advance lines | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	141.3		151.4
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses				(2.4)	(2.2)	(2.2)
Financing Receivable, Loan in Process	1.5		1.8
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	71.6		75.9
Nonaccrual loans	0.0		0.0
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		0.8
Credit card | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.4		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	0.5		0.6
Credit card | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	70.1		74.1
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses	(12.9)	(13.7)	(23.3)	(21.8)	(20.3)	(21.1)
Financing Receivable, Loan in Process	11.3		15.3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	965.6		1,058.5
Nonaccrual loans	3.2		3.0
Total consumer loans | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	7.6		10.9
Total consumer loans | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.8		3.1
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	0.9		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	951.1		1,040.2
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses	(48.4)	(49.5)	(54.9)	(58.5)	(59.9)	(31.6)
Financing Receivable, Loan in Process	18.9		10.9
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	2,925.1		2,882.6
Nonaccrual loans	12.0		10.2
Commercial | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.3		7.1
Commercial | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	16.1		1.7
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	0.5		2.1
Commercial | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2,894.2		2,861.5
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Allowance for credit losses	(3.2)	$ (2.7)	(3.2)	$ (4.0)	$ (6.5)	$ (0.3)
Financing Receivable, Loan in Process	1.0		3.1
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	690.5		708.3
Nonaccrual loans	2.9		8.7
Agricultural | 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
Agricultural | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		2.2
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	0.3		0.1
Agricultural | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	686.6		696.5
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0		0.0
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	5.0		9.2
Nonaccrual loans	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	5.0		9.2
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	54.4		68.8
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	18,226.4		18,114.8
Nonaccrual loans	81.4		59.2
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	21.1		45.8
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	30.1		16.6
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	3.2		6.4
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	$ 18,090.6		$ 17,986.8