Investment Securities (Amortized Cost Table and Fair Value of Held to Maturity and Available for Sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	$ 3,851,202	$ 3,562,537	$ 3,494,000
Available-for-sale Securities, Gross Unrealized Gain at End of Period	30,747	35,382	40,816
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(43,440)	(52,631)	(56,396)
Available-for-sale Securities Fair Value	3,838,509	3,545,288	3,478,420
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	2,787,513	2,903,609	3,052,118
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	17,579	22,108	18,601
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(44,972)	(49,797)	(68,853)
Held-to-maturity Securities, Fair Value	2,760,120	2,875,920	3,001,866
US Treasury Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	5,004
Available-for-sale Securities, Gross Unrealized Gain at End of Period	1
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0
Available-for-sale Securities Fair Value	5,005
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	0	5,000	5,000
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	0	0	5
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	0	0	0
Held-to-maturity Securities, Fair Value	0	5,000	5,005
US Government Agencies Debt Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	2,500	2,500
Available-for-sale Securities, Gross Unrealized Gain at End of Period	10	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	(18)
Available-for-sale Securities Fair Value	2,510	2,482
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	25,000	25,000	25,000
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	0	0	0
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(323)	(537)	(707)
Held-to-maturity Securities, Fair Value	24,677	24,463	24,293
U.S. States and political subdivisions debt securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	203,449	221,052	233,228
Available-for-sale Securities, Gross Unrealized Gain at End of Period	5,191	6,756	8,600
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(1,023)	(466)	(1,023)
Available-for-sale Securities Fair Value	207,617	227,342	240,805
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	529,441	517,824	549,850
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	8,104	12,645	10,072
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(1,942)	(191)	(1,253)
Held-to-maturity Securities, Fair Value	535,603	530,278	558,669
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	947,347	951,839	998,698
Available-for-sale Securities, Gross Unrealized Gain at End of Period	18,068	22,377	24,965
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(4,563)	(3,218)	(5,489)
Available-for-sale Securities Fair Value	960,852	970,998	1,018,174
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	555,273	580,727	624,605
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	6,919	7,495	6,825
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(2,940)	(3,045)	(4,933)
Held-to-maturity Securities, Fair Value	559,252	585,177	626,497
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	171,842	104,176	86,870
Available-for-sale Securities, Gross Unrealized Gain at End of Period	643	598	351
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(2,147)	(1,371)	(1,523)
Available-for-sale Securities Fair Value	170,338	103,403	85,698
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	57,462	58,143	56,020
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	412	281	176
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(219)	(329)	(445)
Held-to-maturity Securities, Fair Value	57,655	58,095	55,751
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	1,968,918	1,698,015	1,627,871
Available-for-sale Securities, Gross Unrealized Gain at End of Period	4,779	4,777	5,009
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(24,308)	(26,225)	(34,849)
Available-for-sale Securities Fair Value	1,949,389	1,676,567	1,598,031
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	1,267,321	1,368,534	1,450,479
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	445	718	114
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(35,025)	(38,875)	(54,263)
Held-to-maturity Securities, Fair Value	1,232,741	1,330,377	1,396,330
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	5	7	8
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0	0	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	0	0
Available-for-sale Securities Fair Value	5	7	8
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 Basis	227,889	222,876	182,151
Available-for-sale Securities, Gross Unrealized Gain at End of Period	1,254	863	1,008
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(705)	(1,405)	(1,126)
Available-for-sale Securities Fair Value	228,438	222,334	182,033
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	263,741	257,642	248,974
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	1,004	557	641
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(4,523)	(6,768)	(7,252)
Held-to-maturity Securities, Fair Value	260,222	251,431	242,363
Collateralized Loan Obligations
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	259,743	297,446	297,334
Available-for-sale Securities, Gross Unrealized Gain at End of Period	801	11	883
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(2,463)	(9,613)	(4,252)
Available-for-sale Securities Fair Value	258,081	287,844	293,965
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	0
Corporate debt securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	61,681	61,652	61,624
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0	0	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(8,231)	(10,315)	(8,134)
Available-for-sale Securities Fair Value	53,450	51,337	53,490
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	89,275	90,739	92,190
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	695	412	768
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	0	(52)	0
Held-to-maturity Securities, Fair Value	89,970	91,099	92,958
Debt Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	3,848,378	3,559,563	3,487,784
Available-for-sale Securities, Gross Unrealized Gain at End of Period	30,747	35,382	40,816
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(43,440)	(52,631)	(56,396)
Available-for-sale Securities Fair Value	3,835,685	3,542,314	3,472,204
Marketable Securities and Investments Held at Cost
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	2,824	2,974	2,935
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0	0	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	0	0
Available-for-sale Securities Fair Value		2,974	2,935
Other Security Investments
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	0		3,281
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 Securities Fair Value	0		3,281
Held-to-maturity Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	2,824	2,974	6,216
Available-for-sale Securities, Gross Unrealized Gain at End of Period	0	0	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	0	0
Available-for-sale Securities Fair Value	$ 2,824	$ 2,974	$ 6,216