Investment Securities (Contractual Maturities Debt Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost	$ 142
Held-to-maturity, Due in one year or less, Estimated fair value	142
Held-to-maturity, Due after one year through five years, Amortized cost	273
Held-to-maturity, Due after one year through five years, Estimated fair value	274
Held-to-maturity, Due after five years through ten years, Amortized cost	196
Held-to-maturity, Due after five years through ten years, Estimated fair value	196
Held-to-maturity Securities, Debt Maturities, Rolling after Ten Years, Amortized Cost	204
Held-to-maturity, Due after ten years, Estimated fair value	191
Held-to-maturity, Amortized cost	815
Held-to-maturity, fair value	803	$ 850
Available-for-sale, Due in one year or less, Amortized cost	1,884
Available-for-sale, Due in one year or less, Estimated fair value	1,868
Available-for-sale, Due after one year through five years, Amortized cost	5,875
Available-for-sale, Due after one year through five years, Estimated fair value	5,828
Available-for-sale, Due after five years through ten years, Amortized cost	4,904
Available-for-sale, Due after five years through ten years, Estimated fair value	4,860
Available-for-sale, Due after ten years, Amortized cost	2,939
Available-for-sale, Due after ten years, Estimated fair value	2,916
Available-for-sale, Amortized cost	15,602	13,337
Available-for-sale, Estimated fair value	$ 15,472	$ 13,188