Investment Securities (Narrative) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Unrealized Losses	$ (44,267)	$ (40,898)
Held-to-maturity securities, accumulated unrealized holding loss	(59,455)	(73,100)
Securities available-for-sale, at fair value	2,638,037	2,991,091
Securities pledged to secure public funds, carrying value	2,400,000	2,500,000
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Unrealized Losses	(3,814)	(3,503)
Held-to-maturity securities, accumulated unrealized holding loss	(4,824)	(3,824)
Securities available-for-sale, at fair value	306,333	419,706
Agency mortgage-backed securities (agency MBS) [Member]
Schedule of Investments [Line Items]
Unrealized Losses	(19,270)	(19,509)
Held-to-maturity securities, accumulated unrealized holding loss	(37,442)	(41,768)
Securities available-for-sale, at fair value	1,107,841	954,349
Agency commercial mortgage-backed securities (agency CMBS) [Member]
Schedule of Investments [Line Items]
Unrealized Losses	(20,250)	(14,567)
Held-to-maturity securities, accumulated unrealized holding loss	(10,011)	(1,348)
Securities available-for-sale, at fair value	588,026	573,272
Non-agency commercial mortgage-backed securities (non-agency CMBS) [Member]
Schedule of Investments [Line Items]
Unrealized Losses	(74)	(702)
Held-to-maturity securities, accumulated unrealized holding loss	(620)	(411)
Securities available-for-sale, at fair value	361,067	477,365
Collateralized loan obligations (CLO) [Member]
Schedule of Investments [Line Items]
Unrealized Losses	(134)	(519)
Securities available-for-sale, at fair value	209,851	427,390
Trust Preferred Securities Single Issuers [Member]
Schedule of Investments [Line Items]
Unrealized Losses	(46)	(1,748)
Securities available-for-sale, at fair value	7,050	28,633
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Unrealized Losses	(679)	(350)
Securities available-for-sale, at fair value	56,622	109,642
Municipal bonds and notes [Member]
Schedule of Investments [Line Items]
Held-to-maturity securities, accumulated unrealized holding loss	(6,558)	$ (25,749)
Callable at the option of the counterparty [Member]
Schedule of Investments [Line Items]
Securities available-for-sale, at fair value	$ 1,200,000