Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 3,630
Due after one year through five years, Amortized Cost	10,658
Due after five years through ten years, Amortized Cost	6,894
Due after ten years, Amortized Cost	2,268
Total debt securities - available-for-sale, Amortized Cost	28,410	$ 24,330
Due in one year or less, Fair Value	3,628
Due after one year through five years, Fair Value	10,631
Due after five years through ten years, Fair Value	6,932
Due after ten years, Fair Value	2,307
Available-for-sale Securities, Debt Securities	28,424	24,179
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	155
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	155
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	131
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	130
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	103
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	103
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	147
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	148
Held-to-maturity Securities	536	493
Held-to-maturity Securities, Fair Value	536	494
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	3,942
Total debt securities - available-for-sale, Amortized Cost	3,942	2,963
Available-for-sale Securities, Debt Securities	3,911	2,927
Mortgage-backed securities, Fair Value	3,911
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	1,018
Total debt securities - available-for-sale, Amortized Cost	1,018	1,009
Available-for-sale Securities, Debt Securities	1,015	$ 1,002
Mortgage-backed securities, Fair Value	$ 1,015