Investment Securities (Amortized Cost Table and Fair Value of Held to Maturity and Available for Sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Securities available for sale
Available-for-sale Securities, Amortized Cost	$ 4,088,856	$ 4,005,782	$ 3,774,542
Available-for-sale Debt Securities, Gross Unrealized Gains	49,739	18,980	47,287
Available-for-sale Debt Securities, Gross Unrealized Losses	(34,381)	(57,027)	(30,770)
Available-for-sale	4,104,214	3,967,735	3,791,059
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	2,613,700	2,674,093	2,855,174
Held-to-maturity Securities, Gross Unrealized Gains	34,505	28,375	25,071
Held-to-maturity Securities, Gross Unrealized Losses	(15,931)	(43,349)	(31,333)
Held-to-maturity Securities, Fair Value	2,632,274	2,659,119	2,848,912
US Treasury Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost		5,001
Available-for-sale Debt Securities, Gross Unrealized Gains		0
Available-for-sale Debt Securities, Gross Unrealized Losses		(1)
Available-for-sale		5,000
Securities held to maturity
Held-to-maturity Securities, Amortized Cost			5,000
Held-to-maturity Securities, Gross Unrealized Gains			0
Held-to-maturity Securities, Gross Unrealized Losses			0
Held-to-maturity Securities, Fair Value			5,000
US Government Agencies Debt Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	2,500	2,500	2,500
Available-for-sale Debt Securities, Gross Unrealized Gains	20	0	13
Available-for-sale Debt Securities, Gross Unrealized Losses	0	(2)	0
Available-for-sale	2,520	2,498	2,513
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	25,000	25,000	25,000
Held-to-maturity Securities, Gross Unrealized Gains	26	19	0
Held-to-maturity Securities, Gross Unrealized Losses	0	0	(178)
Held-to-maturity Securities, Fair Value	25,026	25,019	24,822
U.S. States and political subdivisions debt securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	169,281	188,829	208,800
Available-for-sale Debt Securities, Gross Unrealized Gains	3,894	4,170	6,750
Available-for-sale Debt Securities, Gross Unrealized Losses	(34)	(204)	(386)
Available-for-sale	173,141	192,795	215,164
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	558,795	571,738	523,501
Held-to-maturity Securities, Gross Unrealized Gains	17,285	22,180	8,864
Held-to-maturity Securities, Gross Unrealized Losses	(262)	(262)	(1,147)
Held-to-maturity Securities, Fair Value	575,818	593,656	531,218
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost	889,511	900,358	924,453
Available-for-sale Debt Securities, Gross Unrealized Gains	18,398	11,325	24,799
Available-for-sale Debt Securities, Gross Unrealized Losses	(253)	(5,454)	(1,949)
Available-for-sale	907,656	906,229	947,303
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	88,314	507,908	577,278
Held-to-maturity Securities, Gross Unrealized Gains	1,280	4,767	10,745
Held-to-maturity Securities, Gross Unrealized Losses	(266)	(2,999)	(1,677)
Held-to-maturity Securities, Fair Value	89,328	509,676	586,346
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost	181,846	173,912	139,789
Available-for-sale Debt Securities, Gross Unrealized Gains	2,654	220	1,231
Available-for-sale Debt Securities, Gross Unrealized Losses	(252)	(2,023)	(661)
Available-for-sale	184,248	172,109	140,359
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	489,216	64,951	57,818
Held-to-maturity Securities, Gross Unrealized Gains	10,159	294	765
Held-to-maturity Securities, Gross Unrealized Losses	(212)	(574)	(108)
Held-to-maturity Securities, Fair Value	499,163	64,671	58,475
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities available for sale
Available-for-sale Securities, Amortized Cost	2,214,035	2,155,808	1,895,112
Available-for-sale Debt Securities, Gross Unrealized Gains	20,761	2,659	11,305
Available-for-sale Debt Securities, Gross Unrealized Losses	(8,059)	(30,147)	(13,857)
Available-for-sale	2,226,737	2,128,320	1,892,560
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	1,113,901	1,161,340	1,320,215
Held-to-maturity Securities, Gross Unrealized Gains	2,414	75	1,959
Held-to-maturity Securities, Gross Unrealized Losses	(14,257)	(35,881)	(24,846)
Held-to-maturity Securities, Fair Value	1,102,058	1,125,534	1,297,328
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities available for sale
Available-for-sale Securities, Amortized Cost	4	4	6
Available-for-sale Debt Securities, Gross Unrealized Gains	0	0	0
Available-for-sale Debt Securities, Gross Unrealized Losses	0	0	0
Available-for-sale	4	4	6
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	269,197	217,008	241,839
Available-for-sale Debt Securities, Gross Unrealized Gains	3,976	580	2,602
Available-for-sale Debt Securities, Gross Unrealized Losses	(291)	(1,269)	(382)
Available-for-sale	272,882	216,319	244,059
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	251,421	255,359	256,352
Held-to-maturity Securities, Gross Unrealized Gains	2,436	676	1,659
Held-to-maturity Securities, Gross Unrealized Losses	(934)	(3,611)	(3,377)
Held-to-maturity Securities, Fair Value	252,923	252,424	254,634
Collateralized Loan Obligations
Securities available for sale
Available-for-sale Securities, Amortized Cost	297,885	297,831	297,506
Available-for-sale Debt Securities, Gross Unrealized Gains	36	26	587
Available-for-sale Debt Securities, Gross Unrealized Losses	(12,524)	(8,446)	(4,131)
Available-for-sale	285,397	289,411	293,962
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	0
Held-to-maturity Securities, Fair Value	0
Corporate debt securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	61,724	61,710	61,668
Available-for-sale Debt Securities, Gross Unrealized Gains	0	0	0
Available-for-sale Debt Securities, Gross Unrealized Losses	(12,968)	(9,481)	(9,404)
Available-for-sale	48,756	52,229	52,264
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	87,053	87,797	90,010
Held-to-maturity Securities, Gross Unrealized Gains	905	364	1,079
Held-to-maturity Securities, Gross Unrealized Losses	0	(22)	0
Held-to-maturity Securities, Fair Value	87,958	88,139	91,089
Debt Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	4,085,983	4,002,961	3,771,673
Available-for-sale Debt Securities, Gross Unrealized Gains	49,739	18,980	47,287
Available-for-sale Debt Securities, Gross Unrealized Losses	(34,381)	(57,027)	(30,770)
Available-for-sale	4,101,341	3,964,914	3,788,190
Marketable Equity Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	2,873	2,821	2,869
Available-for-sale Debt Securities, Gross Unrealized Gains	0	0	0
Available-for-sale Debt Securities, Gross Unrealized Losses	0	0	0
Available-for-sale	2,873	2,821	2,869
Other Security Investments
Securities available for sale
Available-for-sale Securities, Amortized Cost			0
Available-for-sale Debt Securities, Gross Unrealized Gains			0
Available-for-sale Debt Securities, Gross Unrealized Losses			0
Available-for-sale			0
Equity Securities
Securities available for sale
Available-for-sale Securities, Amortized Cost	2,873		2,869
Available-for-sale Debt Securities, Gross Unrealized Gains	0		0
Available-for-sale Debt Securities, Gross Unrealized Losses	0		0
Available-for-sale	$ 2,873	$ 2,821	$ 2,869