Investments (Details 12) - USD ($) $ in Millions	Sep. 30, 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	$ 188
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	43
Limited partnership interests	6,600	$ 5,814
Fixed income and equity securities
Other details of unrealized loss
Unrealized losses	231	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	133
Unrealized losses	149	344
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	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	28
Unrealized losses	40	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	27
Unrealized losses	42	103
Unrealized losses having loss of less than twenty percent, less than 12 months	16
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	15
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	2
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	12
Equity-method limited partnerships
Other details of unrealized loss
Limited partnership interests	5,260	4,530
Carrying value
Other details of unrealized loss
Cost method limited partnerships	1,339	1,282
Carrying value | Cost-method Investments
Other details of unrealized loss
Cost method limited partnerships	$ 1,340	$ 1,280