Investment Securities (Amortized Cost Table) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Securities available for sale
Available-for-sale Securities, Amortized Cost	$ 3,562,537	$ 3,329,117
Available-for-sale Securities, Gross Unrealized Gain at End of Period	35,382	34,277
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(52,631)	(90,220)
Available-for-sale	3,545,288	3,273,174
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	2,903,609	2,935,688
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	22,108	6,761
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(49,797)	(118,209)
Held-to-maturity Securities, Fair Value	2,875,920	2,824,240
U.S. Government agencies
Securities available for sale
Available-for-sale Securities, Amortized Cost	2,500
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(18)
Available-for-sale	2,482
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	25,000	25,000
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	0	0
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(537)	(1,348)
Held-to-maturity Securities, Fair Value	24,463	23,652
U.S. States and political subdivisions
Securities available for sale
Available-for-sale Securities, Amortized Cost	221,052	258,787
Available-for-sale Securities, Gross Unrealized Gain at End of Period	6,756	7,376
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(466)	(3,796)
Available-for-sale	227,342	262,367
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	517,824	480,703
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	12,645	5,335
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(191)	(10,459)
Held-to-maturity Securities, Fair Value	530,278	475,579
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost	951,839	962,687
Available-for-sale Securities, Gross Unrealized Gain at End of Period	22,377	21,662
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(3,218)	(14,427)
Available-for-sale	970,998	969,922
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	580,727	569,960
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	7,495	1,108
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(3,045)	(11,617)
Held-to-maturity Securities, Fair Value	585,177	559,451
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost	104,176	72,048
Available-for-sale Securities, Gross Unrealized Gain at End of Period	598	7
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(1,371)	(2,488)
Available-for-sale	103,403	69,567
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	58,143	56,596
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	281	0
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(329)	(1,190)
Held-to-maturity Securities, Fair Value	58,095	55,406
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost	1,698,015	1,566,262
Available-for-sale Securities, Gross Unrealized Gain at End of Period	4,777	4,199
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(26,225)	(52,068)
Available-for-sale	1,676,567	1,518,393
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	1,368,534	1,464,732
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	718	0
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(38,875)	(81,818)
Held-to-maturity Securities, Fair Value	1,330,377	1,382,914
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities available for sale
Available-for-sale Securities, Amortized Cost	7	9
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	0
Available-for-sale	7	9
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	0
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost	222,876	104,152
Available-for-sale Securities, Gross Unrealized Gain at End of Period	863	273
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(1,405)	(2,157)
Available-for-sale	222,334	102,268
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	257,642	240,069
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	557	6
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(6,768)	(11,052)
Held-to-maturity Securities, Fair Value	251,431	229,023
Collateralized Loan Obligations
Securities available for sale
Available-for-sale Securities, Amortized Cost	297,446	297,259
Available-for-sale Securities, Gross Unrealized Gain at End of Period	11	760
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(9,613)	(4,332)
Available-for-sale	287,844	293,687
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	0
Corporate bonds
Securities available for sale
Available-for-sale Securities, Amortized Cost	61,652	61,596
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(10,315)	(10,952)
Available-for-sale	51,337	50,644
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	90,739	93,628
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	412	308
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(52)	(725)
Held-to-maturity Securities, Fair Value	91,099	93,211
Total debt securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	3,559,563	3,322,800
Available-for-sale Securities, Gross Unrealized Gain at End of Period	35,382	34,277
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(52,631)	(90,220)
Available-for-sale	3,542,314	3,266,857
Marketable equity securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	2,974
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0
Available-for-sale	2,974	3,036
Marketable Securities and Investments Held at Cost [Domain]
Securities available for sale
Available-for-sale Securities, Amortized Cost		3,036
Available-for-sale Securities, Gross Unrealized Gain at End of Period		0
Available-for-sale Securities, Gross Unrealized Loss at End of Period		0
Available-for-sale		3,036
Non-marketable equity securities
Securities available for sale
Available-for-sale Securities, Amortized Cost		3,281
Available-for-sale Securities, Gross Unrealized Gain at End of Period		0
Available-for-sale Securities, Gross Unrealized Loss at End of Period		0
Available-for-sale		3,281
Total equity securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	2,974	6,317
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	0
Available-for-sale	2,974	6,317
U.S. Treasury obligations
Securities available for sale
Available-for-sale Securities, Amortized Cost	0
Available-for-sale	0
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	5,000	5,000
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	0	4
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	0	0
Held-to-maturity Securities, Fair Value	$ 5,000	$ 5,004