Loans and Allowance for Loan Losses - Narrative (Details) loan in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Commercial, industrial, commercial real estate or construction loans, held for investment, period of time after payment becomes delinquent that the Company stops accruing interest income and applies subsequent interest payments as principal reductions			90 days
Consumer loans including residential real estate, held for investment, period of time after payment becomes delinquent or when foreclosure proceedings have been initiated that the Company stops accruing interest income			90 days
Number of loan deferrals | loan	13		13
Loan deferrals, amortized cost	$ 1,100.0		$ 1,100.0
Impact to the allowance for loan losses related to modifications classified as TDRs	5.4	$ 6.7	10.6	$ 18.3
Commitments to lend additional funds to borrowers whose terms have been modified in a TDR	100.6		100.6		$ 43.8
Other real estate owned	15.0		15.0		22.0
Loans secured by residential real estate properties for which formal foreclosure proceedings were in process	32.0		32.0		57.0
Other assets
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Accrued interest receivable	232.0		232.0		205.0
Interest Rate Concession
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
TDRs	7.8	7.8	17.3	17.3
Modification of Loan Structure
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
TDRs	113.3	$ 25.0	182.4	$ 57.8
Residential real estate – mortgage
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Other real estate owned	$ 8.0		$ 8.0		$ 14.0
Maximum
Accounts, Notes, Loans and Financing Receivable Including Other Real Estated Owned [Line Items]
Recognition of charge off expenses on consumer loans, period of time loan is past due			120 days
Recognition of charge off expenses on residential loans, period of time loan is past due			180 days
Recognition of charge off expenses on credit card loans, period of time loan is past due			180 days