Investments (Details 12) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	$ 89
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	31
Limited partnership interests	2,946	$ 2,776
Equity method limited partnerships
Other details of unrealized loss
Limited partnership interests	2,360	2,190
Fixed income and equity securities
Other details of unrealized loss
Available for sale securities unrealized losses	(120)	(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	58
Available for sale securities unrealized losses	(74)	(147)
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	16
Equity securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	14
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	17
Available for sale securities unrealized losses	(28)	(53)
Available for sale securities, unrealized losses having loss of less than twenty percent, less than 12 months	8
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	4
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	5
Below investment grade equity 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	11
Carrying value | Cost method limited partnership interests
Other details of unrealized loss
Limited partnership interests	$ 583	$ 591