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, 2017	Sep. 30, 2016	Dec. 31, 2016
Securities Sold under Agreements to Repurchase [Abstract]
Outstanding at end of period	$ 285,409,000	$ 347,804,000	$ 367,052,000
Average amount outstanding	325,230,000	373,474,000
Maximum amount outstanding at any month-end	$ 351,897,000	$ 396,695,000
Weighted average interest rate during period	0.36%	0.41%
Weighted average interest rate at end of period	0.34%	0.42%