Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Due from the Federal Reserve Bank	$ 270,117		$ 521,685
Cash and due from banks	355,732	$ 458,217	490,171
Cash and cash equivalents at end of period	$ 625,849	$ 819,112	$ 1,011,856	$ 1,787,230