Investments (Amortized Cost and Fair Value of Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 2,010
Due after one year through five years, Amortized Cost	6,943
Due after five years through ten years, Amortized Cost	5,016
Due after ten years, Amortized Cost	1,740
Total debt securities - available-for-sale, Amortized Cost	18,637	$ 18,205
Due in one year or less, Fair Value	2,014
Due after one year through five years, Fair Value	7,009
Due after five years through ten years, Fair Value	5,067
Due after ten years, Fair Value	1,776
Available-for-sale Securities, Debt Securities	18,808	18,551
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	2,091
Total debt securities - available-for-sale, Amortized Cost	2,091	2,022
Mortgage-backed securities, Fair Value	2,102
Available-for-sale Securities, Debt Securities	2,102	2,056
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Mortgage-backed securities, Amortized Cost	837
Total debt securities - available-for-sale, Amortized Cost	837	872
Mortgage-backed securities, Fair Value	840
Available-for-sale Securities, Debt Securities	$ 840	$ 880