Investment Securities (Contractual Maturities Debt Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investments [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost	$ 180
Held-to-maturity, Due in one year or less, Estimated fair value	178
Held-to-maturity, Due after one year through five years, Amortized cost	370
Held-to-maturity, Due after one year through five years, Estimated fair value	364
Held-to-maturity, Due after five years through ten years, Amortized cost	159
Held-to-maturity, Due after five years through ten years, Estimated fair value	154
Held-to-maturity Securities, Debt Maturities, Rolling after Ten Years, Amortized Cost	59
Held-to-maturity, Due after ten years, Estimated fair value	56
Held-to-maturity, Amortized cost	768
Held-to-maturity, fair value	752
Available-for-sale, Due in one year or less, Amortized cost	1,906
Available-for-sale, Due in one year or less, Estimated fair value	1,873
Available-for-sale, Due after one year through five years, Amortized cost	4,585
Available-for-sale, Due after one year through five years, Estimated fair value	4,485
Available-for-sale, Due after five years through ten years, Amortized cost	4,645
Available-for-sale, Due after five years through ten years, Estimated fair value	4,544
Available-for-sale, Due after ten years, Amortized cost	4,079
Available-for-sale, Due after ten years, Estimated fair value	3,994
Available-for-sale, Amortized cost	15,215	$ 15,241
Available-for-sale, Estimated fair value	$ 14,896	$ 15,089