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, 2018	Sep. 30, 2017	Dec. 31, 2017
Securities Sold Under Agreements To Repurchase [Abstract]
Outstanding at end of period	$ 319,831,000	$ 285,409,000	$ 384,810,000
Average amount outstanding	342,797,000	325,230,000
Maximum amount outstanding at any month-end	$ 364,001,000	$ 351,897,000
Weighted average interest rate during period	0.52%	0.36%
Weighted average interest rate at end of period	0.74%	0.34%