Investments (Details 11) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	$ 170
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	65
Limited partnerships
Limited partnership interests	2,776	$ 2,295
Equity method limited partnerships
Limited partnerships
Limited partnership interests	2,190	1,770
Fixed income and equity securities
Other details of unrealized loss
Available-for-sale securities, continuous unrealized loss position, accumulated loss	235
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	120
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	27
Below 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	31
Available for sale securities, unrealized losses having loss of less than twenty percent, less than 12 months	17
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	22
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	9
Equity securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	19
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	16
Carrying value
Limited partnerships
Cost method limited partnerships	591	530
Carrying value | Cost method limited partnership interests
Limited partnerships
Cost method limited partnerships	$ 591	$ 530