Investment Securities (Contractual Maturities Debt Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Held-to-maturity, Due in one year or less, Amortized cost	$ 181,416
Held-to-maturity, Due in one year or less, Estimated fair value	181,686
Held-to-maturity, Due after one year through five years, Estimated fair value	276,470
Held-to-maturity, Due after five years through ten years, Estimated fair value	195,541
Held-to-maturity, Due after ten years, Amortized cost	211,068
Held-to-maturity, Due after ten years, Estimated fair value	196,776
Held-to-maturity, Amortized cost	867,904
Held-to-maturity, fair value	850,473	$ 552,088
Available-for-sale, Due in one year or less, Amortized cost	1,548,685
Available-for-sale, Due in one year or less, Estimated fair value	1,532,345
Held To Maturity Securities Debt Maturities One Year Through Five Years, Amortized Cost	276,753
Available-for-sale, Due after one year through five years, Amortized cost	4,971,404
Available-for-sale, Due after one year through five years, Estimated fair value	4,920,537
Held To Maturity Securities Debt Maturities Five Years Through Ten Years, Amortized Cost	198,667
Available-for-sale, Due after five years through ten years, Amortized cost	4,305,373
Available-for-sale, Due after five years through ten years, Estimated fair value	4,251,664
Available-for-sale, Due after ten years, Amortized cost	2,510,847
Available-for-sale, Due after ten years, Estimated fair value	2,483,232
Available-for-sale, Amortized cost	13,336,309
Available-for-sale, Estimated fair value	$ 13,187,778	$ 7,542,480