Investments (Details 12) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investment Holdings
Limited partnership interests	$ 4,874	$ 4,527
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	710
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	389
Fixed income and equity securities
Other details of unrealized loss
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,099	369
Investment grade fixed income securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	338
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	467	158
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	129
Below investment grade fixed income securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	278
Available for sale Securities, Unrealized Losses Having Loss of Less than Twenty Percent, Less than 12 Months	227
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	493	156
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	215
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	16
Equity securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	94
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	139	55
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	45
Equity-method limited partnerships
Investment Holdings
Limited partnership interests	3,720	3,410
Cost-method limited partnership interests
Investment Holdings
Limited partnership interests	$ 1,150	$ 1,120