Investment Securities (Contractual Maturities Debt Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investments [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost	$ 89,799
Held-to-maturity, Due in one year or less, Estimated fair value	90,840
Held-to-maturity, Due after one year through five years, Amortized cost	183,910
Held-to-maturity, Due after one year through five years, Estimated fair value	186,348
Held-to-maturity, Due after five years through ten years, Amortized cost	155,474
Held-to-maturity, Due after five years through ten years, Estimated fair value	159,795
Held-to-maturity Securities, Debt Maturities, Rolling after Ten Years, Amortized Cost	141,686
Held-to-maturity, Due after ten years, Estimated fair value	141,344
Held-to-maturity, Amortized cost	570,869	$ 686,951
Held-to-maturity, fair value	578,327
Available-for-sale, Due in one year or less, Amortized cost	686,832
Available-for-sale, Due in one year or less, Estimated fair value	687,548
Available-for-sale, Due after one year through five years, Amortized cost	1,901,910
Available-for-sale, Due after one year through five years, Estimated fair value	1,902,874
Available-for-sale, Due after five years through ten years, Amortized cost	1,419,932
Available-for-sale, Due after five years through ten years, Estimated fair value	1,418,243
Available-for-sale, Due after ten years, Amortized cost	534,788
Available-for-sale, Due after ten years, Estimated fair value	532,074
Available-for-sale, Amortized cost	4,543,462	3,819,297
Available-for-sale, Estimated fair value	$ 4,540,739	$ 3,708,625