Investment Securities (Amortized Cost Table and Fair Value of Held to Maturity and Available for Sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	$ 3,774,542	$ 3,562,537	$ 3,471,126
Available-for-sale Securities, Gross Unrealized Gain at End of Period	47,287	35,382	34,138
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(30,770)	(52,631)	(72,093)
Available-for-sale Securities Fair Value	3,791,059	3,545,288	3,433,171
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	2,855,174	2,903,609	3,079,620
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	25,071	22,108	3,083
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(31,333)	(49,797)	(92,542)
Held-to-maturity Securities, Fair Value		2,903,609	3,079,620
Held-to-maturity Securities, Fair Value	2,848,912	2,875,920	2,990,161
US Treasury Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	0
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 Securities Fair Value	0
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	5,000	5,000	4,999
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	0	0	6
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	0	0	0
Held-to-maturity Securities, Fair Value			5,005
Held-to-maturity Securities, Fair Value	5,000	5,000
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	13	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	(18)
Available-for-sale Securities Fair Value	2,513	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	(178)	(537)	(1,117)
Held-to-maturity Securities, Fair Value			23,883
Held-to-maturity Securities, Fair Value	24,822	24,463
U.S. States and political subdivisions debt securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	208,800	221,052	245,534
Available-for-sale Securities, Gross Unrealized Gain at End of Period	6,750	6,756	7,541
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(386)	(466)	(2,095)
Available-for-sale Securities Fair Value	215,164	227,342	250,980
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	523,501	517,824	517,221
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	8,864	12,645	159
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(1,147)	(191)	(7,186)
Held-to-maturity Securities, Fair Value			510,194
Held-to-maturity Securities, Fair Value	531,218	530,278
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	924,453	951,839	1,013,085
Available-for-sale Securities, Gross Unrealized Gain at End of Period	24,799	22,377	21,415
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(1,949)	(3,218)	(10,506)
Available-for-sale Securities Fair Value	947,303	970,998	1,023,994
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	577,278	580,727	644,820
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	10,745	7,495	2,311
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(1,677)	(3,045)	(8,584)
Held-to-maturity Securities, Fair Value			638,547
Held-to-maturity Securities, Fair Value	586,346	585,177
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	139,789	104,176	87,258
Available-for-sale Securities, Gross Unrealized Gain at End of Period	1,231	598	36
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(661)	(1,371)	(2,161)
Available-for-sale Securities Fair Value	140,359	103,403	85,133
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	57,818	58,143	56,339
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	765	281	32
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(108)	(329)	(877)
Held-to-maturity Securities, Fair Value			55,494
Held-to-maturity Securities, Fair Value	58,475	58,095
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	1,895,112	1,698,015	1,586,051
Available-for-sale Securities, Gross Unrealized Gain at End of Period	11,305	4,777	3,857
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(13,857)	(26,225)	(41,804)
Available-for-sale Securities Fair Value	1,892,560	1,676,567	1,548,104
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	1,320,215	1,368,534	1,488,641
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	1,959	718	0
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(24,846)	(38,875)	(64,734)
Held-to-maturity Securities, Fair Value			1,423,907
Held-to-maturity Securities, Fair Value	1,297,328	1,330,377
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	6	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	6	7	8
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	0
Held-to-maturity Securities, Fair Value	0
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	241,839	222,876	173,951
Available-for-sale Securities, Gross Unrealized Gain at End of Period	2,602	863	331
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(382)	(1,405)	(1,813)
Available-for-sale Securities Fair Value	244,059	222,334	172,469
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	256,352	257,642	249,688
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	1,659	557	138
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(3,377)	(6,768)	(9,749)
Held-to-maturity Securities, Fair Value			240,077
Held-to-maturity Securities, Fair Value	254,634	251,431
Collateralized Loan Obligations
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	297,506	297,446	297,293
Available-for-sale Securities, Gross Unrealized Gain at End of Period	587	11	958
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(4,131)	(9,613)	(3,692)
Available-for-sale Securities Fair Value	293,962	287,844	294,559
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	61,668	61,652	61,610
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	(9,404)	(10,315)	(10,022)
Available-for-sale Securities Fair Value	52,264	51,337	51,588
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	90,010	90,739	92,912
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	1,079	412	437
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	0	(52)	(295)
Held-to-maturity Securities, Fair Value			93,054
Held-to-maturity Securities, Fair Value	91,089	91,099
Debt Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	3,771,673	3,559,563	3,464,790
Available-for-sale Securities, Gross Unrealized Gain at End of Period	47,287	35,382	34,138
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(30,770)	(52,631)	(72,093)
Available-for-sale Securities Fair Value	3,788,190	3,542,314	3,426,835
Marketable Securities and Investments Held at Cost
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	2,869	2,974	3,055
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	3,055
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	2,869	2,974	6,336
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	$ 2,869	$ 2,974	$ 6,336