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	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	$ 342,480	$ 362,294	$ 308,189
U.S. Treasury [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	342,480
Overnight and Continuous [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	342,480
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	342,480
Up to 30 Days [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
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	0
30-90 Days [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
30-90 Days [Member] | U.S. Treasury [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
Greater Than 90 days [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
Greater Than 90 days [Member] | U.S. Treasury [Member]
Assets Sold Under Agreements To Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	$ 0