Investment Securities (by Maturity buckets and narrative) (Details 2) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity[Abstract]
Due in One Year or Less	$ 90,025,000
Due After One Year Through Five Years	349,765,000
Due After Five Years Through Ten Years	541,657,000
Due After Ten Years	95,040,000
Amortized Cost, Total	1,076,487,000
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Due in One Year or Less	90,886,000
Due After One Year Through Five Years	351,442,000
Due After Five Years Through Ten Years	554,898,000
Due After Ten Years	102,294,000
Fair Value, Total	1,099,520,000
Available-for-sale Securities, Amortized Cost	2,236,092,000	2,262,807,000
Available-for-sale Securities, Fair Value	2,271,186,000	2,289,190,000
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturities (Abstract]
Due in One Year or Less	89,760,000
Due After One Year Through Five Years	439,937,000
Due After Five Years Through Ten Years	224,823,000
Due After Ten Years	175,870,000
Amortized Cost, Total	930,390,000
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Due in One Year or Less	90,447,000
Due After One Year Through Five Years	444,139,000
Due After Five Years Through Ten Years	232,586,000
Due After Ten Years	185,211,000
Fair Value, Total	952,383,000
Held-to-maturity Securities, Amortized Cost	4,306,353,000	4,466,679,000
Held-to-Maturity Securities, Fair Value	4,378,007,000	4,504,495,000
Carrying value of investment securities which are pledged	2,600,000,000	2,800,000,000
Debt Securities Issued by Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	289,895,000
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	294,544,000
Mortgage-Backed Securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	869,710,000
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	877,122,000
Available-for-sale Securities, Amortized Cost	869,710,000	915,696,000
Available-for-sale Securities, Fair Value	877,122,000	919,129,000
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-maturity Securities, Debt Maturities, Debt Maturities, without Single Maturity Date, Amortized Cost	3,375,963,000
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Fair Value	3,425,624,000
Held-to-maturity Securities, Amortized Cost	3,375,963,000	3,551,667,000
Held-to-Maturity Securities, Fair Value	3,425,624,000	3,576,508,000
Residential - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	417,962,000
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	427,391,000
Available-for-sale Securities, Amortized Cost	417,962,000	452,493,000
Available-for-sale Securities, Fair Value	427,391,000	462,436,000
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-maturity Securities, Debt Maturities, Debt Maturities, without Single Maturity Date, Amortized Cost	2,706,237,000
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Fair Value	2,746,426,000
Held-to-maturity Securities, Amortized Cost	2,706,237,000	2,862,369,000
Held-to-Maturity Securities, Fair Value	2,746,426,000	2,887,140,000
Residential - U.S. Government Sponsored Enterprises
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	279,708,000
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	283,574,000
Available-for-sale Securities, Amortized Cost	279,708,000	276,390,000
Available-for-sale Securities, Fair Value	283,574,000	278,461,000
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-maturity Securities, Debt Maturities, Debt Maturities, without Single Maturity Date, Amortized Cost	361,273,000
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Fair Value	367,418,000
Held-to-maturity Securities, Amortized Cost	361,273,000	379,365,000
Held-to-Maturity Securities, Fair Value	367,418,000	382,985,000
Commercial - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	172,040,000
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value	166,157,000
Available-for-sale Securities, Amortized Cost	172,040,000	186,813,000
Available-for-sale Securities, Fair Value	166,157,000	178,232,000
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-maturity Securities, Debt Maturities, Debt Maturities, without Single Maturity Date, Amortized Cost	308,453,000
Held-to-maturity Securities, Debt Maturities, without Single Maturity Date, Fair Value	311,780,000
Held-to-maturity Securities, Amortized Cost	308,453,000	309,933,000
Held-to-Maturity Securities, Fair Value	$ 311,780,000	$ 306,383,000