Investment Securities (Maturities Table) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	4 years 1 month 16 days
Securities Available for Sale
One year or less	$ 54,373
Available for Sale Securities Debt Maturities Maturity Period of One Year or Less Weighted Average Yield	2.71%
Over one year through five years	2,904,756
Available for Sale Securities Debt Maturities Maturity Period over One Year Through Five Years Weighted Average Yield	2.15%
Over five years through ten years	750,801
Available for Sale Securities Debt Maturities Maturity Period over Five Years Through Ten Years Weighted Average Yield	2.93%
Over ten years	78,260
Available for Sale Securities Debt Maturities Maturity Period over Ten Years Weighted Average Yield	1.89%
Available-for-sale	3,791,059	3,545,288	3,433,171
Amortized Cost	3,774,542	3,562,537	3,471,126
Weighted-Average Yield	2.31%
Securities Held to Maturity
One year or less	78,410
Over one year through five years	2,123,533
Held to Maturity Securities Debt Maturities Maturity Period over One Year Through Five Years Weighted Average Yield	2.05%
Over five years through ten years	486,282
Held to Maturity Securities Debt Maturities Maturity Period over Five Years Through Ten Years Weighted Average Yield	1.88%
Over ten years	160,687
Held to Maturity Securities Debt Maturities Maturity Period Weighted Average Yield	5.50%
Held-to-maturity Securities, Fair Value	2,848,912	2,875,920	2,990,161
Held-to-maturity Securities, Carrying Amount		2,903,609	3,079,620
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,855,174	2,903,609	3,079,620
Weighted Average Yield	2.31%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	4 years 7 months 3 days
Held to Maturity Securities Debt Maturities Maturity Period over Ten Years Weighted Average Yield	3.21%
US Government Agencies Debt Securities
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 2 months 3 days
Securities Available for Sale
One year or less	0
Over one year through five years	0
Over five years through ten years	2,513
Over ten years	0
Available-for-sale	2,513	2,482
Amortized Cost	2,500	2,500
Weighted-Average Yield	1.25%
Securities Held to Maturity
One year or less	0
Over one year through five years	0
Over five years through ten years	24,822
Over ten years	0
Held-to-maturity Securities, Fair Value	24,822	24,463
Held-to-maturity Securities, Carrying Amount			23,883
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	25,000	25,000	25,000
Weighted Average Yield	1.43%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	4 years 6 months 30 days
US Treasury Securities
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	1 year 0 months 0 days
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	0
Available-for-sale	0
Amortized Cost	0
Weighted-Average Yield	0.00%
Securities Held to Maturity
One year or less	5,000
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	5,000	5,000
Held-to-maturity Securities, Carrying Amount			5,005
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	5,000	5,000	4,999
Weighted Average Yield	0.31%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	0 years 0 months 1 day
U.S. States and political subdivisions debt securities
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	5 years 9 months 15 days
Securities Available for Sale
One year or less	14,933
Over one year through five years	74,124
Over five years through ten years	100,111
Over ten years	25,996
Available-for-sale	215,164	227,342	250,980
Amortized Cost	208,800	221,052	245,534
Weighted-Average Yield	5.24%
Securities Held to Maturity
One year or less	57,026
Over one year through five years	93,443
Over five years through ten years	220,062
Over ten years	160,687
Held-to-maturity Securities, Fair Value	531,218	530,278
Held-to-maturity Securities, Carrying Amount			510,194
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	523,501	517,824	517,221
Weighted Average Yield	4.70%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	9 years 3 months 12 days
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 8 months 1 day
Securities Available for Sale
One year or less	1,742
Over one year through five years	789,867
Over five years through ten years	155,694
Over ten years	0
Available-for-sale	947,303	970,998	1,023,994
Amortized Cost	924,453	951,839	1,013,085
Weighted-Average Yield	2.55%
Securities Held to Maturity
One year or less	0
Over one year through five years	472,347
Over five years through ten years	113,999
Over ten years	0
Held-to-maturity Securities, Fair Value	586,346	585,177
Held-to-maturity Securities, Carrying Amount			638,547
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	577,278	580,727	644,820
Weighted Average Yield	2.17%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	4 years 0 months 22 days
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	4 years 7 months 29 days
Securities Available for Sale
One year or less	15,907
Over one year through five years	44,313
Over five years through ten years	80,139
Over ten years	0
Available-for-sale	140,359	103,403	85,133
Amortized Cost	139,789	104,176	87,258
Weighted-Average Yield	2.05%
Securities Held to Maturity
One year or less	16,384
Over one year through five years	31,832
Over five years through ten years	10,259
Over ten years	0
Held-to-maturity Securities, Fair Value	58,475	58,095
Held-to-maturity Securities, Carrying Amount			55,494
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	57,818	58,143	56,339
Weighted Average Yield	2.15%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 3 months 8 days
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 7 months 7 days
Securities Available for Sale
One year or less	14,287
Over one year through five years	1,827,726
Over five years through ten years	50,547
Over ten years	0
Available-for-sale	1,892,560	1,676,567	1,548,104
Amortized Cost	1,895,112	1,698,015	1,586,051
Weighted-Average Yield	1.92%
Securities Held to Maturity
One year or less	0
Over one year through five years	1,274,419
Over five years through ten years	22,909
Over ten years	0
Held-to-maturity Securities, Fair Value	1,297,328	1,330,377
Held-to-maturity Securities, Carrying Amount			1,423,907
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	1,320,215	1,368,534	1,488,641
Weighted Average Yield	1.60%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 9 months 4 days
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	1 year 11 months 27 days
Securities Available for Sale
One year or less	0
Over one year through five years	6
Over five years through ten years	0
Over ten years	0
Available-for-sale	6	7	8
Amortized Cost	6	7	8
Weighted-Average Yield	3.39%
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, Carrying Amount	0
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	0
Weighted Average Yield	0.00%
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 9 months 26 days
Securities Available for Sale
One year or less	0
Over one year through five years	168,720
Over five years through ten years	75,339
Over ten years	0
Available-for-sale	244,059	222,334	172,469
Amortized Cost	241,839	222,876	173,951
Weighted-Average Yield	1.89%
Securities Held to Maturity
One year or less	0
Over one year through five years	160,403
Over five years through ten years	94,231
Over ten years	0
Held-to-maturity Securities, Fair Value	254,634	251,431
Held-to-maturity Securities, Carrying Amount			240,077
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	256,352	257,642	249,688
Weighted Average Yield	2.27%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	4 years 4 months 30 days
Collateralized Loan Obligations
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	6 years 0 months 22 days
Securities Available for Sale
One year or less	7,504
Over one year through five years	0
Over five years through ten years	286,458
Over ten years	0
Available-for-sale	293,962	287,844	294,559
Amortized Cost	297,506	297,446	297,293
Weighted-Average Yield	2.70%
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, Amortized Cost before Other than Temporary Impairment	0
Weighted Average Yield	0.00%
Corporate debt securities
Schedule of Available-for-Sale and Held-to-Maturity Securities [Line Items]
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	12 years 6 months 23 days
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	52,264
Available-for-sale	52,264	51,337	51,588
Amortized Cost	61,668	61,652	61,610
Weighted-Average Yield	0.98%
Securities Held to Maturity
One year or less	0
Over one year through five years	91,089
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	91,089	91,099
Held-to-maturity Securities, Carrying Amount			93,054
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	90,010	90,739	92,912
Weighted Average Yield	2.23%
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	2 years 9 months 11 days
Debt Securities
Securities Available for Sale
Available-for-sale	3,788,190	3,542,314	3,426,835
Amortized Cost	3,771,673	3,559,563	3,464,790
Fair Value, Inputs, Level 3
Securities Available for Sale
Available-for-sale	346,231	339,187	349,425
Securities Held to Maturity
Held-to-maturity Securities, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | U.S. States and political subdivisions debt securities
Securities Available for Sale
Available-for-sale	0	0	0
Fair Value, Inputs, Level 3 | Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
Available-for-sale	0	0	0
Fair Value, Inputs, Level 3 | Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
Available-for-sale	0	0	0
Fair Value, Inputs, Level 3 | Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
Available-for-sale	0	0	0
Fair Value, Inputs, Level 3 | Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities Available for Sale
Available-for-sale	5	6	7
Fair Value, Inputs, Level 3 | Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
Available-for-sale	0	0	0
Fair Value, Inputs, Level 3 | Collateralized Loan Obligations
Securities Available for Sale
Available-for-sale		287,844	294,559
Securities Held to Maturity
Held-to-maturity Securities, Carrying Amount	0
Fair Value, Inputs, Level 3 | Corporate debt securities
Securities Available for Sale
Available-for-sale		$ 51,337	$ 51,588