Securities Sold Under Agreements to Repurchase - Schedule of Securities Sold under Agreements to Repurchase and Related Weighted-Average Interest Rates (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Securities Sold under Agreements to Repurchase [Abstract]
Securities sold under agreements to repurchase	$ 399,141,000	$ 337,551,000	$ 327,782,000
Average amount outstanding during the period	355,039,000	344,294,000
Maximum amount outstanding at any month-end during the period	$ 399,141,000	$ 367,884,000
Weighted average interest rate during period	0.27%	0.80%
Weighted average interest rate at end of period	0.12%	0.74%	0.53%