Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and due from banks	$ 3,697,018	$ 2,672,290
Securities purchased under agreements to resell	119,181	247,876
Other short-term investment securities	2,942	2,909
Total cash and cash equivalents	$ 3,819,141	$ 2,923,075	$ 3,555,571	$ 2,545,750