Loans and Allowance for Credit Losses for Loans - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non covered loans net of unearned discount and deferred loan fees	$ 108,600,000		$ 95,800,000
Accrued interest	92,400,000		90,200,000
Transfer of loans to loans held for sale	0	$ 30,020,000
Sales of loans	$ 0	$ 0
Days past due to place on non-accrual status	90 days	90 days
Number of consecutive months for performing restructured loans to be put on accrual status	6 months
TDRs not reported as non-accrual loans	$ 67,100,000		57,400,000
Non-performing TDRs	91,500,000		92,800,000
Specific reserves for loan losses	3,400,000	$ 7,900,000
Loan charge-offs related to loans modified as TDRs	$ 5,100,000	791,000
Loan term modification, deferral period	90 days
Loan term modification, deferral period extension	90 days
Amount from modified loans	$ 284,000,000		361,000,000
Other real estate owned	4,500,000		5,100,000
Residential real estate properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other real estate owned	413,000		1,000,000.0
In formal foreclosure proceedings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage and consumer loans secured by residential real estate properties	$ 1,800,000		$ 1,900,000
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of loans to loans held for sale		$ 30,000,000.0