Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Due from the Federal Reserve Bank	$ 182,047		$ 356,410
Cash and due from banks	366,169	$ 422,117	354,184
Cash and cash equivalents at end of period	$ 548,216	$ 1,063,967	$ 710,594	$ 819,112