Investments - Unrealized loss and Limited partnerships (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 cost or amortized cost	$ 269
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	26
Limited partnership interests	6,740	$ 5,814
Fixed income and equity securities
Other details of unrealized loss
Available-for-sale securities, continuous unrealized loss position, accumulated loss	295	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	219
Available-for-sale securities, continuous unrealized loss position, accumulated loss	232	344
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	13
Equity securities
Other details of unrealized loss
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	11
Available-for-sale securities, continuous unrealized loss position, accumulated loss	12	85
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	1
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	39
Available-for-sale securities, continuous unrealized loss position, accumulated loss	51	103
Available for sale securities, unrealized losses having loss of less than twenty percent, less than 12 months	22
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	12
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
EMA limited partnerships
Other details of unrealized loss
Limited partnership interests	5,410	4,530
Cost-method limited partnership interests
Other details of unrealized loss
Limited partnership interests	$ 1,330	$ 1,280