Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	[1]
Cash and Cash Equivalents [Abstract]
Cash and due from banks	$ 2,672,290	$ 2,476,588
Securities purchased under agreements to resell	247,876	64,028
Other short-term investment securities	2,909	5,134
Total cash and cash equivalents	$ 2,923,075	$ 2,545,750	$ 1,503,257	$ 1,811,014

[1] Cash and cash equivalents at December 31, 2014 included $15.0 million recognized in assets held-for-sale in conjunction with the SVBIF sale transaction. On April 13, 2015 we received net proceeds of $39.3 million consisting of the sales price of $48.6 million less $9.3 million of cash and cash equivalents held by SVBIF that were sold.