Investments (Details 12) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	$ (116)
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	(69)
Limited partnership interests	2,124	2,024
Equity method limited partnerships
Other details of unrealized loss
Limited partnership interests	1,610	1,520
Investment grade fixed income securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	(52)
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	(39)
Below Investment Grade Equity Securities [Member]
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	(28)
Below investment grade fixed income securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	(2)
Unrealized losses related to securities with unrealized loss position greater than 20% of cost or amortized cost, unrealized loss position of 12 or more consecutive months	6
Carrying value | Cost method limited partnership interests
Other details of unrealized loss
Limited partnership interests	$ 515	$ 508