Securities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sales of securities available for sale	$ 315,890,000	$ 411,981,000	$ 95,525,000
Gross realized gains from securities	7,255,000	4,636,000	581,000
Gross realized losses from securities	274,000	1,418,000	3,000
Accrued interest on securities available for sale	42,600,000
ACL related to AFS Securities	0
Sales of securities held to maturity	0	0	0
Accrued interest on securities held to maturity	5,300,000
Unrealized gain (loss) on trading securities	13,000	1,000	(18,000)
Securities sold not yet purchased	2,200,000		$ 14,600,000
Collateral Dependent Assets [Member] | U.S. Government and Other Public Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	7,800,000,000	5,800,000,000
Collateral Dependent Assets [Member] | Federal Reserve Discount Window [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	$ 371,500,000	$ 481,200,000