Securities Sold Under Agreements to Repurchase - Schedule of Securities Sold under Agreements to Repurchase and Related Weighted-Average Interest Rates (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Securities Sold under Agreements to Repurchase [Abstract]
Securities sold under agreements to repurchase	$ 431,166,000	$ 327,782,000
Average amount outstanding	375,961,000	342,654,000
Maximum amount outstanding at any month-end	$ 438,039,000	$ 367,884,000
Weighted average interest rate during period	0.23%	0.73%
Weighted average interest rate at end of period	0.12%	0.53%