Loans and Allowance for Credit Losses for Loans - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non covered loans net of unearned discount and deferred loan fees	$ 73,000,000		$ 73,000,000		$ 95,800,000
Accrued interest	83,400,000		83,400,000		90,200,000
Transfer of loans to loans held for sale			0	$ 30,020,000
Sales of loans	0	$ 0	$ 0	0
Number of consecutive months for performing restructured loans to be put on accrual status			6 months
TDRs not reported as non-accrual loans	64,800,000		$ 64,800,000		57,400,000
Non-performing TDRs	116,700,000		116,700,000		92,800,000
Specific reserves for loan losses	8,200,000	18,700,000	8,200,000	18,700,000
Loan charge-offs related to loans modified as TDRs	206,000	$ 1,900,000	6,000,000	5,600,000
Amount from modified loans	98,600,000		98,600,000		361,000,000
Other real estate owned	4,000,000		4,000,000		5,100,000
Residential real estate properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other real estate owned	0		0		1,000,000
In formal foreclosure proceedings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage and consumer loans secured by residential real estate properties	$ 3,000,000		$ 3,000,000		$ 1,900,000
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of loans to loans held for sale				$ 30,000,000