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, 2017	Mar. 31, 2016	Dec. 31, 2016
Securities Sold under Agreements to Repurchase [Abstract]
Outstanding at end of period	$ 345,550,000	$ 379,060,000	$ 367,052,000
Average amount outstanding	331,400,000	386,044,000
Maximum amount outstanding at any month-end	$ 345,550,000	$ 396,695,000
Weighted average interest rate during period	0.31%	0.39%
Weighted average interest rate at end of period	0.35%	0.39%