Investments - Total unrealized losses and limited partnerships narrative (Details) - USD ($) $ in Millions	Dec. 31, 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	$ 81
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	20
Limited partnerships
Limited partnership interests	3,147	$ 2,776
Equity method limited partnerships
Limited partnerships
Limited partnership interests	2,540	2,190
Fixed income and equity securities
Other details of unrealized loss
Available-for-sale securities, continuous unrealized loss position, accumulated loss	101
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	61
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	13
Equity securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	3
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 having loss of less than twenty percent, less than 12 months	7
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	7
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
Carrying value
Limited partnerships
Cost method limited partnerships	611	591
Carrying value | Cost method limited partnership interests
Limited partnerships
Cost method limited partnerships	$ 611	$ 591