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, 2017	Jun. 30, 2016	Dec. 31, 2016
Securities Sold under Agreements to Repurchase [Abstract]
Outstanding at end of period	$ 298,094,000	$ 354,123,000	$ 367,052,000
Average amount outstanding	330,285,000	382,441,000
Maximum amount outstanding at any month-end	$ 351,897,000	$ 396,695,000
Weighted average interest rate during period	0.36%	0.40%
Weighted average interest rate at end of period	0.41%	0.43%