Investment Securities (Maturities Table) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Securities Available for Sale
One year or less	$ 7,239
Available for Sale Securities Debt Maturities Maturity Period of One Year or Less Weighted Average Yield	3.29%
Over one year through five years	$ 208,258
Available for Sale Securities Debt Maturities Maturity Period over One Year Through Five Years Weighted Average Yield	3.80%
Over five years through ten years	$ 567,425
Available for Sale Securities Debt Maturities Maturity Period over Five Years Through Ten Years Weighted Average Yield	3.06%
Over ten years	$ 3,318,419
Available for Sale Securities Debt Maturities Maturity Period over Ten Years Weighted Average Yield	2.11%
Available-for-sale	$ 4,104,214	$ 3,967,735	$ 3,791,059
Amortized Cost	$ 4,088,856	4,005,782	3,774,542
Weighted-Average Yield	2.33%
Securities Held to Maturity
One year or less	$ 88,093
Held to Maturity Securities Debt Maturities Maturity Period over One Year Through Five Years Weighted Average Yield	2.21%
Over one year through five years	$ 365,180
Held to Maturity Securities Debt Maturities Maturity Period over Five Years Through Ten Years Weighted Average Yield	2.45%
Over five years through ten years	$ 318,693
Held to Maturity Securities Debt Maturities Maturity Period over Ten Years Weighted Average Yield	3.83%
Over ten years	$ 1,860,308
Held to Maturity Securities Debt Maturities Maturity Period Weighted Average Yield	2.07%
Held-to-maturity Securities, Fair Value	$ 2,632,274	2,659,119	2,848,912
Held-to-maturity Securities, Carrying Amount	$ 2,613,700	2,674,093	2,855,174
Weighted Average Yield	2.34%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 12 months 7 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 12 months 29 days
US Government Agencies Debt Securities
Securities Available for Sale
One year or less	$ 0
Over one year through five years	2,520
Over five years through ten years	0
Over ten years	0
Available-for-sale	2,520	2,498	2,513
Amortized Cost	$ 2,500	2,500	2,500
Weighted-Average Yield	1.25%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	25,026
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	25,026	25,019	24,822
Held-to-maturity Securities, Carrying Amount	$ 25,000	25,000	25,000
Weighted Average Yield	1.43%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	2 years 8 months 2 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 12 months 29 days
US Treasury Securities
Securities Available for Sale
Available-for-sale		5,000
Amortized Cost		5,001
Securities Held to Maturity
Held-to-maturity Securities, Fair Value			5,000
Held-to-maturity Securities, Carrying Amount			5,000
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	6 months
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	1 day
U.S. States and political subdivisions debt securities
Securities Available for Sale
One year or less	$ 7,120
Over one year through five years	79,686
Over five years through ten years	66,049
Over ten years	20,286
Available-for-sale	173,141	192,795	215,164
Amortized Cost	$ 169,281	188,829	208,800
Weighted-Average Yield	5.17%
Securities Held to Maturity
One year or less	$ 65,484
Over one year through five years	157,086
Over five years through ten years	202,251
Over ten years	150,997
Held-to-maturity Securities, Fair Value	575,818	593,656	531,218
Held-to-maturity Securities, Carrying Amount	$ 558,795	571,738	523,501
Weighted Average Yield	4.22%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	2 years 5 months 24 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	5 years 4 months 11 days
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 119
Over one year through five years	61,444
Over five years through ten years	52,312
Over ten years	793,781
Available-for-sale	907,656	906,229	947,303
Amortized Cost	$ 889,511	900,358	924,453
Weighted-Average Yield	2.40%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	36,437
Over five years through ten years	52,891
Over ten years	0
Held-to-maturity Securities, Fair Value	89,328	509,676	586,346
Held-to-maturity Securities, Carrying Amount	$ 88,314	507,908	577,278
Weighted Average Yield	2.02%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 9 months 23 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 12 months 29 days
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 0
Over one year through five years	32,937
Over five years through ten years	122,070
Over ten years	29,241
Available-for-sale	184,248	172,109	140,359
Amortized Cost	$ 181,846	173,912	139,789
Weighted-Average Yield	2.15%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	22,567
Over ten years	476,596
Held-to-maturity Securities, Fair Value	499,163	64,671	58,475
Held-to-maturity Securities, Carrying Amount	$ 489,216	64,951	57,818
Weighted Average Yield	2.15%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 12 months 14 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	2 years 7 months 17 days
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 0
Over one year through five years	16,015
Over five years through ten years	11,655
Over ten years	2,199,067
Available-for-sale	2,226,737	2,128,320	1,892,560
Amortized Cost	$ 2,214,035	2,155,808	1,895,112
Weighted-Average Yield	2.04%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	1,102,058
Held-to-maturity Securities, Fair Value	1,102,058	1,125,534	1,297,328
Held-to-maturity Securities, Carrying Amount	$ 1,113,901	1,161,340	1,320,215
Weighted Average Yield	1.60%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 8 months 9 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 8 months 9 days
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities Available for Sale
One year or less	$ 0
Over one year through five years	3
Over five years through ten years	0
Over ten years	1
Available-for-sale	4	4	6
Amortized Cost	$ 4	4	6
Weighted-Average Yield	2.60%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	0
Held-to-maturity Securities, Carrying Amount	$ 0
Weighted Average Yield	0.00%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 12 months 25 days
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 0
Over one year through five years	15,653
Over five years through ten years	110,696
Over ten years	146,533
Available-for-sale	272,882	216,319	244,059
Amortized Cost	$ 269,197	217,008	241,839
Weighted-Average Yield	2.21%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	81,282
Over five years through ten years	40,984
Over ten years	130,657
Held-to-maturity Securities, Fair Value	252,923	252,424	254,634
Held-to-maturity Securities, Carrying Amount	$ 251,421	255,359	256,352
Weighted Average Yield	2.00%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 9 months 30 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	4 years 1 month 27 days
Collateralized Loan Obligations
Securities Available for Sale
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	204,643
Over ten years	80,754
Available-for-sale	285,397	289,411	293,962
Amortized Cost	$ 297,885	297,831	297,506
Weighted-Average Yield	3.07%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	0
Held-to-maturity Securities, Carrying Amount	$ 0
Weighted Average Yield	0.00%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	6 years 5 months 24 days
Corporate debt securities
Securities Available for Sale
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	48,756
Available-for-sale	48,756	52,229	52,264
Amortized Cost	$ 61,724	61,710	61,668
Weighted-Average Yield	1.34%
Securities Held to Maturity
One year or less	$ 22,609
Over one year through five years	65,349
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	87,958	88,139	91,089
Held-to-maturity Securities, Carrying Amount	$ 87,053	87,797	90,010
Weighted Average Yield	2.26%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	11 years 12 months 22 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	2 years 3 months 12 days
Debt Securities
Securities Available for Sale
Available-for-sale	$ 4,101,341	3,964,914	3,788,190
Amortized Cost	$ 4,085,983	$ 4,002,961	$ 3,771,673