Loans and Allowance for Credit Losses for Loans - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) component	Sep. 30, 2019	Sep. 30, 2020 USD ($) SecurityLoan component	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-covered loans net of unearned premiums and deferred loan fees	$ 116,200,000		$ 116,200,000		$ 12,600,000
Accrued interest	$ 117,900,000		117,900,000		86,300,000
Transfer of loans to loans held for sale			30,020,000	$ 302,951,000
Sales of loans			$ 0	$ 0
Days past due to place on non-accrual status	90 days	90 days	90 days	90 days
Number of consecutive months for performing restructured loans to be put on accrual status			6 years
TDRs not reported as non-accrual loans	$ 58,100,000		$ 58,100,000		73,000,000.0
Non-performing TDRs	96,800,000		96,800,000		65,100,000
Specific reserves for loan losses			18,700,000	$ 29,600,000
Loan charge-offs related to loans modified as TDRs	1,900,000		$ 5,600,000	2,000,000.0
Number of modified loans | SecurityLoan			1,400
Amount from modified loans			$ 1,100,000,000
Other real estate owned	$ 7,700,000		$ 7,700,000		9,400,000
Number of basic components | component	2		2
Forecast period			2 years
Historical loss experience period			1 year
Allowance for unfunded credit commitments	$ 10,296,000		$ 10,296,000		2,845,000
Residential real estate properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Other real estate owned	799,000		799,000		2,100,000
In formal foreclosure proceedings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage and consumer loans secured by residential real estate properties	$ 2,000,000.0		2,000,000.0		2,800,000
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Transfer of loans to loans held for sale			$ 30,000,000.0	$ 303,000,000.0
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired loans threshold					$ 250,000