Securities Sold Under Agreements to Repurchase - Schedule of Securities Sold under Agreements to Repurchase and Related Weighted-Average Interest Rates (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Securities Sold Under Agreements To Repurchase [Abstract]
Outstanding at end of period	$ 342,480,000	$ 308,189,000	$ 362,294,000
Average amount outstanding	361,261,000	342,682,000
Maximum amount outstanding at any month-end	$ 367,884,000	$ 336,319,000
Weighted average interest rate during period	0.74%	0.42%
Weighted average interest rate at end of period	0.79%	0.48%