Securities Sold Under Agreements to Repurchase - Schedule of Remaining Contractual Maturity of Secured Borrowings and Class of Collateral Pledged Under Repurchase Agreements (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	$ 384,810	$ 367,052
U.S. Treasury [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	384,810
Overnight and Continuous [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	359,810
Overnight and Continuous [Member] | U.S. Treasury [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	359,810
Up to 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	25,000
Up to 30 Days [Member] | U.S. Treasury [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	$ 25,000