Loans Held for Investment - Schedule of Loans by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Deferred loan fees and costs	$ (9.8)	$ (12.3)
Loans held for investment, net of deferred fees and costs	16,945.0	9,331.7
Allowance for credit losses	(247.2)	(122.3)	$ (136.6)	$ (144.3)
Net loans held for investment	16,697.8	9,209.4
Commercial real estate
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	7,805.7	3,971.5
Allowance for credit losses	(125.0)	(43.9)	(52.2)	(54.8)
Financing Receivable, Loan in Process	9.7	2.7
Commercial real estate | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	7.2	1.1
Commercial real estate | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.2	1.0
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	0.6
Commercial real estate | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	7,758.6	3,960.8
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	344.8	247.8
Allowance for credit losses	(0.5)	(0.5)	(1.2)	(1.3)
Financing Receivable, Loan in Process	2.7	0.2
Land acquisition & development | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.6	0.2
Land acquisition & development | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.1	0.0
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	0.0	0.0
Land acquisition & development | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	338.2	246.9
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	406.0	262.0
Allowance for credit losses	(3.3)	(2.4)	(1.4)	(1.6)
Financing Receivable, Loan in Process	2.6	4.2
Residential | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.6	4.2
Residential | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.0	0.0
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	403.4	257.8
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	844.8	498.0
Allowance for credit losses	(10.2)	(6.0)	(7.2)	(7.3)
Financing Receivable, Loan in Process	0.4	0.0
Commercial | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.4	0.0
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	844.4	498.0
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,595.6	1,007.8
Financing Receivable, Loan in Process	5.7	4.4
Total construction loans | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	5.6	4.4
Total construction loans | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.1	0.0
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	0.0	0.0
Total construction loans | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1,586.0	1,002.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	1,997.5	1,538.2
Allowance for credit losses	(20.1)	(14.6)	(12.9)	(12.8)
Financing Receivable, Loan in Process	6.1	3.9
Residential | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	4.4	3.0
Residential | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.6	0.8
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	0.1	0.1
Residential | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1,984.3	1,531.4
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	833.6	213.9
Allowance for credit losses	(5.4)	(1.9)	(2.8)	(2.7)
Financing Receivable, Loan in Process	4.4	2.1
Agricultural | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.1	1.9
Agricultural | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	3.9	0.2
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	0.4	0.0
Agricultural | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	790.8	206.9
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	12,232.4	6,731.4
Allowance for credit losses	(164.5)	(69.3)	(77.7)	(80.5)
Financing Receivable, Loan in Process	25.9	13.1
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	17.3	10.4
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	7.8	2.0
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	0.8	0.7
Total real estate loans | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	12,119.7	6,701.8
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	739.6	737.6
Allowance for credit losses	(13.5)	(14.3)	(16.0)	(16.7)
Financing Receivable, Loan in Process	6.3	6.9
Indirect consumer | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	5.2	5.1
Indirect consumer | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.9	1.4
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	0.2	0.4
Indirect consumer | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	731.5	729.0
Direct and advance lines
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	142.5	129.2
Allowance for credit losses	(4.6)	(4.6)	(4.8)	(4.6)
Financing Receivable, Loan in Process	0.8	0.8
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	0.5	0.5
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	0.3	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	0.0	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	141.6	128.3
Credit card
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	73.5	64.9
Allowance for credit losses	(2.3)	(2.2)	(1.5)	(2.6)
Financing Receivable, Loan in Process	0.9	1.3
Credit card | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	0.3	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	0.2
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	0.5
Credit card | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	72.6	63.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	955.6	931.7
Allowance for credit losses	(20.4)	(21.1)	(22.3)	(23.9)
Financing Receivable, Loan in Process	8.0	9.0
Total consumer loans | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	6.0	6.2
Total consumer loans | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	1.4	1.8
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	0.6	1.0
Total consumer loans | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	945.7	920.9
Commercial
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	3,017.9	1,475.5
Allowance for credit losses	(58.2)	(31.6)	(36.0)	(39.2)
Financing Receivable, Loan in Process	7.1	6.7
Commercial | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2.0	4.9
Commercial | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	3.8	0.7
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	1.3	1.1
Commercial | Financial Asset, Not Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	2,995.6	1,463.8
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	744.3	203.9
Allowance for credit losses	(4.1)	(0.3)	$ (0.6)	$ (0.7)
Financing Receivable, Loan in Process	16.1	0.7
Agricultural | 30 to 59 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	11.4	0.7
Agricultural | 60 to 89 Days Past Due
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Financing Receivable, Loan in Process	4.7	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	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	713.2	201.6
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	4.6	1.5
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	4.6	1.5
Loans And Leases Held For Investment [Member]
Loans and Leases Receivable Recorded Investment, Past Due [Line Items]
Loans held for investment	16,954.8	9,344.0
Financing Receivable, Loan in Process	57.1	29.5
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	36.7	22.2
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	17.7	4.5
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	2.7	2.8
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	$ 16,778.8	$ 9,289.6