Investment Securities (Amortized Cost Table and Fair Value of Held to Maturity and Available for Sale Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	$ 3,912,767	$ 3,562,537	$ 3,494,793
Available-for-sale Securities, Gross Unrealized Gain at End of Period	41,039	35,382	32,807
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(28,082)	(52,631)	(64,610)
Available-for-sale Securities Fair Value	3,925,724	3,545,288	3,462,990
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	2,728,638	2,903,609	3,002,262
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	27,401	22,108	17,796
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(27,128)	(49,797)	(72,886)
Held-to-maturity Securities, Fair Value	2,728,911	2,875,920	2,947,172
US Treasury Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	5,002
Available-for-sale Securities, Gross Unrealized Gain at End of Period	3
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	3
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period		0	0
Held-to-maturity Securities, Fair Value	0	5,000	5,003
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	16	0
Available-for-sale Securities, Gross Unrealized Loss at End of Period	0	(18)
Available-for-sale Securities Fair Value	2,516	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	(41)	(537)	(756)
Held-to-maturity Securities, Fair Value	24,959	24,463	24,244
U.S. States and political subdivisions debt securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	196,396	221,052	229,528
Available-for-sale Securities, Gross Unrealized Gain at End of Period	4,825	6,756	8,146
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(411)	(466)	(607)
Available-for-sale Securities Fair Value	200,810	227,342	237,067
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	550,769	517,824	539,014
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	12,271	12,645	12,256
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(977)	(191)	(452)
Held-to-maturity Securities, Fair Value	562,063	530,278	550,818
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	927,715	951,839	983,038
Available-for-sale Securities, Gross Unrealized Gain at End of Period	19,624	22,377	20,395
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(1,715)	(3,218)	(7,046)
Available-for-sale Securities Fair Value	945,624	970,998	996,387
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	529,095	580,727	602,220
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	9,732	7,495	4,608
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(1,249)	(3,045)	(5,992)
Held-to-maturity Securities, Fair Value	537,578	585,177	600,836
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	153,834	104,176	86,485
Available-for-sale Securities, Gross Unrealized Gain at End of Period	1,722	598	167
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(480)	(1,371)	(1,888)
Available-for-sale Securities Fair Value	155,076	103,403	84,764
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	64,605	58,143	65,704
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	1,162	281	28
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(11)	(329)	(676)
Held-to-maturity Securities, Fair Value	65,756	58,095	65,056
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	2,023,275	1,698,015	1,635,780
Available-for-sale Securities, Gross Unrealized Gain at End of Period	12,200	4,777	3,512
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(12,516)	(26,225)	(38,975)
Available-for-sale Securities Fair Value	2,022,959	1,676,567	1,600,317
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	1,210,743	1,368,534	1,415,421
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	1,509	718	66
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(22,432)	(38,875)	(56,260)
Held-to-maturity Securities, Fair Value	1,189,820	1,330,377	1,359,227
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	5	7	7
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	7
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	241,788	222,876	195,512
Available-for-sale Securities, Gross Unrealized Gain at End of Period	2,462	863	298
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(290)	(1,405)	(1,793)
Available-for-sale Securities Fair Value	243,960	222,334	194,017
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	259,888	257,642	258,436
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	2,005	557	346
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	(2,418)	(6,768)	(8,570)
Held-to-maturity Securities, Fair Value	259,475	251,431	250,212
Collateralized Loan Obligations
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	297,776	297,446	297,389
Available-for-sale Securities, Gross Unrealized Gain at End of Period	187	11	289
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(5,512)	(9,613)	(7,257)
Available-for-sale Securities Fair Value	292,451	287,844	290,421
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	61,695	61,652	61,638
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	(7,158)	(10,315)	(7,044)
Available-for-sale Securities Fair Value	54,537	51,337	54,594
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	88,538	90,739	91,467
Held-to-maturity Securities, Unrecognized Holding Gain End of Period	722	412	489
Held-to-Maturity Securities, Unrecognized Holding Loss End of Period	0	(52)	(180)
Held-to-maturity Securities, Fair Value	89,260	91,099	91,776
Debt Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	3,909,986	3,559,563	3,489,377
Available-for-sale Securities, Gross Unrealized Gain at End of Period	41,039	35,382	32,807
Available-for-sale Securities, Gross Unrealized Loss at End of Period	(28,082)	(52,631)	(64,610)
Available-for-sale Securities Fair Value	3,922,943	3,542,314	3,457,574
Marketable Securities and Investments Held at Cost
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	2,781	2,974	2,916
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	2,916
Other Security Investments
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis			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			0
Available-for-sale Securities Fair Value			2,500
Held-to-maturity Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost Basis	2,781	2,974	5,416
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	$ 2,781	$ 2,974	$ 5,416