Loans and Allowance for Credit Losses - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) portfolio	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) portfolio	Sep. 30, 2019 USD ($)	Sep. 30, 2020 segment	Sep. 30, 2020 application	Sep. 30, 2020 USD ($)	Sep. 30, 2020	Dec. 31, 2019 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of loan portfolios | portfolio	4		4
Number of loan segments	7		7		7
Accrued interest receivable, loans							$ 59,500,000
Allowance for credit loss, net recoveries	$ 3,000,000.0
Loan placed on nonaccrual when past due, number of days			90 days
Loan participations							874,000,000.0
Loan participations sold							392,900,000
Loan participations retained							481,100,000
Troubled debt restructuring term			6 months
Nonaccrual period for loans			90 days
Minimum number of days for loan charge off to be recorded			120 days
Maximum number of days for loan charge off to be recorded			180 days
Financing receivable TDRs included with non-accrual loans							7,700,000		$ 13,800,000
Financing receivable troubled debt restructurings specific reserves							300,000		900,000
Unfunded commitments on TDRs							800,000		$ 2,300,000
Provision expense for (recoveries from) credit losses	0	$ 1,437,000	$ 39,495,000	$ 3,483,000
Charge-offs	(3,259,000)	(32,000)	$ (1,351,000)	139,000
Number of days for a loan to be considered to be in payment default			90 days
Troubled Debt Restructurings During Period
Financing Receivable, Allowance for Credit Losses [Line Items]
Provision expense for (recoveries from) credit losses	(300,000)			700,000
Charge-offs	0			0
Maximum
Financing Receivable, Allowance for Credit Losses [Line Items]
Value of small commercial loans on nonaccrual status or 90 days or more delinquent			$ 250,000
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
PPP, number of applications processed and approved (over) | application						9,700
PPP, amount of funding							$ 1,500,000,000
Provision expense for (recoveries from) credit losses	(3,289,000)	(373,000)	2,087,000	488,000
Charge-offs	(1,326,000)	(12,000)	(632,000)	89,000
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of risk-based capital								220.00%
Regulatory guideline limit								300.00%
Provision expense for (recoveries from) credit losses	4,173,000	465,000	30,696,000	(845,000)
Charge-offs	$ (1,993,000)	$ (13,000)	$ (741,000)	$ 58,000