Investments - Realized Capital Gains and Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Realized capital gains and losses by asset type
Realized capital gains and losses	$ 48	$ 19	$ 24	$ 14
Fixed income securities
Realized capital gains and losses by asset type
Realized capital gains and losses	(8)	4	(26)	(9)
Mortgage loans
Realized capital gains and losses by asset type
Realized capital gains and losses	0	1	2	1
Equity securities
Realized capital gains and losses by asset type
Realized capital gains and losses	56	7	50	3
Limited partnership interests
Realized capital gains and losses by asset type
Realized capital gains and losses	(4)	11	(12)	34
Derivatives
Realized capital gains and losses by asset type
Realized capital gains and losses	4	(4)	11	(13)
Other
Realized capital gains and losses by asset type
Realized capital gains and losses	$ 0	$ 0	$ (1)	$ (2)