Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Due in one year or less, Amortized Cost	$ 2,007
Due after one year through five years, Amortized Cost	6,944
Due after five years through ten years, Amortized Cost	4,900
Due after ten years, Amortized Cost	1,732
Total debt securities - available-for-sale, Amortized Cost	18,662
Due in one year or less, Fair Value	2,019
Due after one year through five years, Fair Value	7,052
Due after five years through ten years, Fair Value	4,962
Due after ten years, Fair Value	1,773
Available-for-sale Securities, Debt Securities	18,871
U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities, Amortized Cost	2,261
Mortgage-backed securities, Fair Value	2,239
Available-for-sale Securities, Debt Securities	2,239	2,218
Non-U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities, Amortized Cost	818
Mortgage-backed securities, Fair Value	826
Available-for-sale Securities, Debt Securities	$ 826	$ 702