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	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	$ 319,831	$ 384,810	$ 285,409
U.S. Treasury [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	319,831
Overnight and Continuous [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	319,831
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	$ 319,831