Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 3,586
Due after one year through five years, Amortized Cost	11,945
Due after five years through ten years, Amortized Cost	7,076
Due after ten years, Amortized Cost	2,563
Total debt securities - available-for-sale, Amortized Cost	31,178	$ 28,410
Due in one year or less, Fair Value	3,580
Due after one year through five years, Fair Value	11,794
Due after five years through ten years, Fair Value	6,942
Due after ten years, Fair Value	2,548
Debt Securities, Available-for-sale	30,716	28,424
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	138
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	193
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	103
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	154
Held-to-maturity securities, Amortized Cost	588	536
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value	138
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	191
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value	103
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	154
Debt Securities, Held-to-maturity, Fair Value	586	536
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	4,723
Total debt securities - available-for-sale, Amortized Cost	4,723	3,942
Mortgage-backed securities, Fair Value	4,594
Debt Securities, Available-for-sale	4,594	3,911
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	1,285
Total debt securities - available-for-sale, Amortized Cost	1,285	1,018
Mortgage-backed securities, Fair Value	1,258
Debt Securities, Available-for-sale	$ 1,258	$ 1,015