Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and due from banks	$ 2,241,287	$ 2,672,290
Securities purchased under agreements to resell	375,180	247,876
Other short-term investment securities	2,917	2,909
Total cash and cash equivalents	$ 2,619,384	$ 2,923,075	$ 3,795,679	$ 2,545,750