Investment Securities (Summary Of Investment Securities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities [Abstract]
Amortized Cost	$ 2,675,131	$ 3,015,763
Unrealized Gains	7,173	16,226
Unrealized Losses	(44,267)	(40,898)
Fair Value	2,638,037	2,991,091
Held-to-maturity Securities [Abstract]
Amortized Cost	4,487,392	4,160,658
Unrealized Gains	28,413	37,567
Unrealized Losses	(59,455)	(73,100)
Fair Value	4,456,350	4,125,125
US Treasury Bill Securities [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	1,247	734
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	1,247	734
Collateralized Mortgage Obligations [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	308,989	419,865
Unrealized Gains	1,158	3,344
Unrealized Losses	(3,814)	(3,503)
Fair Value	306,333	419,706
Held-to-maturity Securities [Abstract]
Amortized Cost	260,114	339,455
Unrealized Gains	664	1,977
Unrealized Losses	(4,824)	(3,824)
Fair Value	255,954	337,608
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	1,124,960	969,460
Unrealized Gains	2,151	4,398
Unrealized Losses	(19,270)	(19,509)
Fair Value	1,107,841	954,349
Held-to-maturity Securities [Abstract]
Amortized Cost	2,569,735	2,317,449
Unrealized Gains	16,989	26,388
Unrealized Losses	(37,442)	(41,768)
Fair Value	2,549,282	2,302,069
Agency commercial mortgage-backed securities (agency CMBS) [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	608,276	587,776
Unrealized Gains	0	63
Unrealized Losses	(20,250)	(14,567)
Fair Value	588,026	573,272
Held-to-maturity Securities [Abstract]
Amortized Cost	696,566	547,726
Unrealized Gains	0	694
Unrealized Losses	(10,011)	(1,348)
Fair Value	686,555	547,072
Commercial Mortgage Backed Securities [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	358,984	473,974
Unrealized Gains	2,157	4,093
Unrealized Losses	(74)	(702)
Fair Value	361,067	477,365
Held-to-maturity Securities [Abstract]
Amortized Cost	249,273	298,538
Unrealized Gains	2,175	4,107
Unrealized Losses	(620)	(411)
Fair Value	250,828	302,234
Collateralized loan obligations (CLO) [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	209,075	425,083
Unrealized Gains	910	2,826
Unrealized Losses	(134)	(519)
Fair Value	209,851	427,390
Trust Preferred Securities Single Issuers [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	7,096	30,381
Unrealized Gains	0	0
Unrealized Losses	(46)	(1,748)
Fair Value	7,050	28,633
Corporate Debt Securities [Member]
Available-for-sale Securities [Abstract]
Amortized Cost	56,504	108,490
Unrealized Gains	797	1,502
Unrealized Losses	(679)	(350)
Fair Value	56,622	109,642
Municipal Bonds [Member]
Held-to-maturity Securities [Abstract]
Amortized Cost	711,381	655,813
Unrealized Gains	8,584	4,389
Unrealized Losses	(6,558)	(25,749)
Fair Value	713,407	634,453
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Held-to-maturity Securities [Abstract]
Amortized Cost	323	1,677
Unrealized Gains	1	12
Unrealized Losses	0	0
Fair Value	$ 324	$ 1,689

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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