Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Due from the FRB	$ 1,323,539	$ 641,850
Cash and due from banks	392,723	422,117
Cash and cash equivalents at end of year	$ 1,716,262	$ 1,063,967	$ 819,112	$ 1,787,230