Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and due from banks	$ 2,670,473	$ 2,672,290
Securities purchased under agreements to resell	37,379	247,876
Other short-term investment securities	4,249	2,909
Total cash and cash equivalents	$ 2,712,101	$ 2,923,075	$ 3,854,244	$ 2,545,750