Investment Securities (Contractual Maturities Debt Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investments [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost	$ 108
Held-to-maturity, Due in one year or less, Estimated fair value	108
Held-to-maturity, Due after one year through five years, Amortized cost	276
Held-to-maturity, Due after one year through five years, Estimated fair value	278
Held-to-maturity, Due after five years through ten years, Amortized cost	207
Held-to-maturity, Due after five years through ten years, Estimated fair value	210
Held-to-maturity Securities, Debt Maturities, Rolling after Ten Years, Amortized Cost	184
Held-to-maturity, Due after ten years, Estimated fair value	178
Held-to-maturity, Amortized cost	775
Held-to-maturity, fair value	774	$ 850
Available-for-sale, Due in one year or less, Amortized cost	1,990
Available-for-sale, Due in one year or less, Estimated fair value	1,983
Available-for-sale, Due after one year through five years, Amortized cost	5,771
Available-for-sale, Due after one year through five years, Estimated fair value	5,752
Available-for-sale, Due after five years through ten years, Amortized cost	4,787
Available-for-sale, Due after five years through ten years, Estimated fair value	4,784
Available-for-sale, Due after ten years, Amortized cost	2,769
Available-for-sale, Due after ten years, Estimated fair value	2,767
Available-for-sale, Amortized cost	15,317	13,337
Available-for-sale, Estimated fair value	$ 15,286	$ 13,188