Investment Securities (Contractual Maturities Debt Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investments [Abstract]
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 242
Held-to-maturity, Due in one year or less, Estimated fair value	242
Held-to-maturity, Due after one year through five years, Estimated fair value	341
Held-to-maturity, Due after five years through ten years, Estimated fair value	133
Held-to-maturity, Due after ten years, Estimated fair value	51
Held-to-maturity, Amortized cost	774
Held-to-maturity, fair value	767
Available-for-sale, Due in one year or less, Amortized cost	1,955
Available-for-sale, Due in one year or less, Estimated fair value	1,916
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	344
Available-for-sale, Due after one year through five years, Amortized cost	5,806
Available-for-sale, Due after one year through five years, Estimated fair value	5,690
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	135
Available-for-sale, Due after five years through ten years, Amortized cost	4,608
Available-for-sale, Due after five years through ten years, Estimated fair value	4,520
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	53
Available-for-sale, Due after ten years, Amortized cost	2,671
Available-for-sale, Due after ten years, Estimated fair value	2,611
Available-for-sale, Amortized cost	15,040	$ 15,241
Available-for-sale, Estimated fair value	$ 14,737	$ 15,089