Investment Securities (Maturities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Securities Available for Sale
One year or less	$ 13,640
Over one year through five years	190,332
Over five years through ten years	590,610
Over ten years	3,170,332
Available-for-sale	$ 3,967,735	$ 3,545,288
Weighted Average Yield, one year or less	1.99%
Weighted Average Yield, over one year through five years	3.84%
Weighted Average Yield, over five years through ten years	3.02%
Weighted Average Yield, over ten years	2.09%
Available-for-sale Securities, Amortized Cost Basis	$ 4,005,782	3,562,537
Weighted-Average Yield	2.31%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	4 years 1 month 17 days
Securities Held to Maturity
One year or less	$ 63,383
Over one year through five years	373,078
Over five years through ten years	309,660
Over ten years	$ 1,912,998
Weighted Average Yield, over one year through five years	1.99%
Weighted Average Yield, over five years through ten years	2.34%
Weighted Average Yield, over ten years	3.79%
Weighted Average Yield, Total	2.12%
Held-to-maturity Securities, Fair Value	$ 2,659,119	2,875,920
Held-to-maturity	$ 2,674,093	2,903,609
Held to Maturity Securities Debt Maturities Weighted Average Yield	2.33%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	3 years 11 months 16 days
U.S. Government agencies
Securities Available for Sale
One year or less	$ 0
Over one year through five years	2,498
Over five years through ten years	0
Over ten years	0
Available-for-sale	2,498	2,482
Available-for-sale Securities, Amortized Cost Basis	$ 2,500	2,500
Weighted-Average Yield	1.25%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	2 years 5 months 1 day
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	25,019
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	25,019	24,463
Held-to-maturity	$ 25,000	25,000
Held to Maturity Securities Debt Maturities Weighted Average Yield	1.43%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	29 days
U.S. Treasury obligations
Securities Available for Sale
One year or less	$ 5,000
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Available-for-sale	5,000
Available-for-sale Securities, Amortized Cost Basis	$ 5,001
Weighted-Average Yield	0.28%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	0 years
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	5,000
Held-to-maturity	$ 0	5,000
Held to Maturity Securities Debt Maturities Weighted Average Yield	0.00%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	0 years
U.S. States and political subdivisions
Securities Available for Sale
One year or less	$ 8,640
Over one year through five years	78,968
Over five years through ten years	83,928
Over ten years	21,259
Available-for-sale	192,795	227,342
Available-for-sale Securities, Amortized Cost Basis	$ 188,829	221,052
Weighted-Average Yield	5.19%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	2 years 2 months 1 day
Securities Held to Maturity
One year or less	$ 63,383
Over one year through five years	144,134
Over five years through ten years	216,331
Over ten years	169,808
Held-to-maturity Securities, Fair Value	593,656	530,278
Held-to-maturity	$ 571,738	517,824
Held to Maturity Securities Debt Maturities Weighted Average Yield	4.01%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	4 years 9 months 29 days
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 0
Over one year through five years	37,010
Over five years through ten years	71,478
Over ten years	797,741
Available-for-sale	906,229	970,998
Available-for-sale Securities, Amortized Cost Basis	$ 900,358	951,839
Weighted-Average Yield	2.39%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	3 years 9 months 11 days
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	23,421
Over ten years	486,255
Held-to-maturity Securities, Fair Value	509,676	585,177
Held-to-maturity	$ 507,908	580,727
Held to Maturity Securities Debt Maturities Weighted Average Yield	2.15%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	4 years 15 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	14,825
Over five years through ten years	136,728
Over ten years	20,556
Available-for-sale	172,109	103,403
Available-for-sale Securities, Amortized Cost Basis	$ 173,912	104,176
Weighted-Average Yield	2.03%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	4 years 6 months 7 days
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	35,953
Over five years through ten years	28,718
Over ten years	0
Held-to-maturity Securities, Fair Value	64,671	58,095
Held-to-maturity	$ 64,951	58,143
Held to Maturity Securities Debt Maturities Weighted Average Yield	2.55%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	2 years 10 months 28 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	10,922
Over five years through ten years	23,697
Over ten years	2,093,701
Available-for-sale	2,128,320	1,676,567
Available-for-sale Securities, Amortized Cost Basis	$ 2,155,808	1,698,015
Weighted-Average Yield	2.04%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	3 years 10 months 21 days
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	1,125,534
Held-to-maturity Securities, Fair Value	1,125,534	1,330,377
Held-to-maturity	$ 1,161,340	1,368,534
Held to Maturity Securities Debt Maturities Weighted Average Yield	1.60%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	3 years 9 months 4 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	4
Over five years through ten years	0
Over ten years	0
Available-for-sale	4	7
Available-for-sale Securities, Amortized Cost Basis	$ 4	7
Weighted-Average Yield	3.07%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	10 months 24 days
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	$ 0
Held to Maturity Securities Debt Maturities Weighted Average Yield	0.00%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	0 years
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	46,105
Over five years through ten years	67,299
Over ten years	102,915
Available-for-sale	216,319	222,334
Available-for-sale Securities, Amortized Cost Basis	$ 217,008	222,876
Weighted-Average Yield	2.11%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	4 years
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	79,833
Over five years through ten years	41,190
Over ten years	131,401
Held-to-maturity Securities, Fair Value	252,424	251,431
Held-to-maturity	$ 255,359	257,642
Held to Maturity Securities Debt Maturities Weighted Average Yield	2.34%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	4 years 15 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	207,480
Over ten years	81,931
Available-for-sale	289,411	287,844
Available-for-sale Securities, Amortized Cost Basis	$ 297,831	297,446
Weighted-Average Yield	2.77%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	6 years 6 months 4 days
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	$ 0
Held to Maturity Securities Debt Maturities Weighted Average Yield	0.00%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	0 years
Corporate bonds
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	52,229
Available-for-sale	52,229	51,337
Available-for-sale Securities, Amortized Cost Basis	$ 61,710	61,652
Weighted-Average Yield	1.24%
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity	11 years 9 months 22 days
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	88,139
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	88,139	91,099
Held-to-maturity	$ 87,797	90,739
Held to Maturity Securities Debt Maturities Weighted Average Yield	2.28%
Held to Maturity Securities Debt Maturities Weighted Average Maturity	2 years 7 days
Total debt securities
Securities Available for Sale
Available-for-sale	$ 3,964,914	3,542,314
Available-for-sale Securities, Amortized Cost Basis	$ 4,002,961	$ 3,559,563