Investments (Details 12) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	$ 314
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	73
Limited partnership interests	5,982	$ 5,814
Fixed income and equity securities
Other details of unrealized loss
Unrealized losses	387	532
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	232
Unrealized losses	258	344
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	26
Equity securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	34
Unrealized losses	55	85
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	48
Unrealized losses	74	103
Unrealized losses having loss of less than twenty percent, less than 12 months	28
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	26
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	17
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 cost or amortized cost	21
Equity-method limited partnerships
Other details of unrealized loss
Limited partnership interests	4,690	4,530
Carrying value
Other details of unrealized loss
Cost method limited partnerships	1,293	1,282
Carrying value | Cost-method Investments
Other details of unrealized loss
Cost method limited partnerships	$ 1,290	$ 1,280