Investment Securities (Contractual Maturities Debt Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investments [Abstract]
Held-to-maturity Securities, Debt Maturities, Next Rolling Twelve Months, Amortized Cost	$ 99
Held-to-maturity, Due in one year or less, Estimated fair value	100
Held-to-maturity, Due after one year through five years, Amortized cost	282
Held-to-maturity, Due after one year through five years, Estimated fair value	284
Held-to-maturity, Due after five years through ten years, Amortized cost	186
Held-to-maturity, Due after five years through ten years, Estimated fair value	188
Held-to-maturity Securities, Debt Maturities, Rolling after Ten Years, Amortized Cost	179
Held-to-maturity, Due after ten years, Estimated fair value	171
Held-to-maturity, Amortized cost	746
Held-to-maturity, fair value	743	$ 850
Available-for-sale, Due in one year or less, Amortized cost	2,296
Available-for-sale, Due in one year or less, Estimated fair value	2,285
Available-for-sale, Due after one year through five years, Amortized cost	5,406
Available-for-sale, Due after one year through five years, Estimated fair value	5,384
Available-for-sale, Due after five years through ten years, Amortized cost	4,784
Available-for-sale, Due after five years through ten years, Estimated fair value	4,779
Available-for-sale, Due after ten years, Amortized cost	2,713
Available-for-sale, Due after ten years, Estimated fair value	2,707
Available-for-sale, Amortized cost	15,199	13,337
Available-for-sale, Estimated fair value	$ 15,155	$ 13,188