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, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	$ 431,166	$ 327,782
U.S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	431,166
Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	431,166
Overnight and Continuous | U.S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	431,166
Up to 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
Up to 30 Days | U.S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
30-90 Days | U.S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
Greater Than 90 days
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	0
Greater Than 90 days | U.S. Treasury
Assets Sold under Agreements to Repurchase [Line Items]
Secured borrowings and class of collateral pledged under repurchase agreements	$ 0