Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Available-for-sale, Debt Maturities, Due in one year or less, Amortized Cost	$ 2,872
Available-for-sale, Debt Maturities, Due in one year or less, Fair Value	2,877
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	139
Held-to-maturity Securities, Debt Maturities, within one year, Fair Value	139
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	8,341
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	8,461
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Net Carrying Amount	175
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	177
Available-for-sale Securities, Debt Maturities, Due after five years through ten years, Amortized Cost	5,234
Available-for-sale Securities, Debt Maturities, Due after five years through ten years, Fair Value	5,438
Held-to-maturity Securities, Debt Maturities, due after five years through ten years, Net Carrying Amount	107
Held-to-maturity Securities, Debt Maturities, due after five years through ten years, Fair Value	107
Available-for-sale Securities, Debt Maturities, Due after ten years, Amortized Cost	2,182
Available-for-sale Securities, Debt Maturities, Due after ten years, Fair Value	2,295
Held-to-maturity Securities, Debt Maturities, after ten years, Net Carrying Amount	88
Held-to-maturity Securities, Debt Maturities, after ten years, Fair Value	89
Available-for-sale Debt Securities, Amortized Cost Basis	22,139	$ 18,539
Available-for-sale Securities, Debt Securities	22,648	18,631
Held-to-maturity securities, Amortized Cost	509	510
Held-to-maturity Securities, Fair Value	512	511
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	2,580
Mortgage-backed securities, Fair Value	2,623
Available-for-sale Debt Securities, Amortized Cost Basis	2,580	2,127
Available-for-sale Securities, Debt Securities	2,623	2,124
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	930
Mortgage-backed securities, Fair Value	954
Available-for-sale Debt Securities, Amortized Cost Basis	930	962
Available-for-sale Securities, Debt Securities	$ 954	$ 956