Securities Sold Under Agreements to Repurchase - Schedule of Securities Sold under Agreements to Repurchase and Related Weighted-Average Interest Rates (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Securities Sold Under Agreements To Repurchase [Abstract]
Outstanding at end of period	$ 334,540,000	$ 347,511,000	$ 362,294,000
Average amount outstanding	346,396,000	337,612,000
Maximum amount outstanding at any month-end	$ 367,884,000	$ 347,511,000
Weighted average interest rate during period	0.78%	0.47%
Weighted average interest rate at end of period	0.86%	0.59%