Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 3,923
Due after one year through five years, Amortized Cost	11,400
Due after five years through ten years, Amortized Cost	6,874
Due after ten years, Amortized Cost	2,343
Total debt securities - available-for-sale, Amortized Cost	30,125	$ 28,410
Due in one year or less, Fair Value	3,915
Due after one year through five years, Fair Value	11,273
Due after five years through ten years, Fair Value	6,766
Due after ten years, Fair Value	2,330
Available-for-sale Securities, Debt Securities	29,742	28,424
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	135
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	202
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	103
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	151
Held-to-maturity securities, Amortized Cost	591	536
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value	135
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	200
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	103
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	152
Held-to-maturity Securities, Fair Value	590	536
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	4,423
Total debt securities - available-for-sale, Amortized Cost	4,423	3,942
Mortgage-backed securities, Fair Value	4,316
Available-for-sale Securities, Debt Securities	4,316	3,911
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	1,162
Total debt securities - available-for-sale, Amortized Cost	1,162	1,018
Mortgage-backed securities, Fair Value	1,142
Available-for-sale Securities, Debt Securities	$ 1,142	$ 1,015