Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Due in one year or less, Amortized Cost	$ 2,540
Due after one year through five years, Amortized Cost	7,219
Due after five years through ten years, Amortized Cost	4,945
Due after ten years, Amortized Cost	1,931
Total debt securities - available-for-sale, Amortized Cost	19,323
Due in one year or less, Fair Value	2,553
Due after one year through five years, Fair Value	7,351
Due after five years through ten years, Fair Value	5,008
Due after ten years, Fair Value	1,937
Available-for-sale Securities, Debt Securities	19,536
U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities, Amortized Cost	2,028
Mortgage-backed securities, Fair Value	2,020
Available-for-sale Securities, Debt Securities	2,020	2,238
Non-U.S. Agency Mortgage-Backed Securities [Member]
Mortgage-backed securities, Amortized Cost	660
Mortgage-backed securities, Fair Value	667
Available-for-sale Securities, Debt Securities	$ 667	$ 574