Investments - Realized Capital Gains and Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Realized capital gains and losses by asset type
Realized capital gains and losses	$ 8	$ (4)	$ (24)	$ (5)
Fixed income securities
Realized capital gains and losses by asset type
Realized capital gains and losses	(12)	(6)	(18)	(13)
Mortgage loans
Realized capital gains and losses by asset type
Realized capital gains and losses	2	0	2	0
Equity securities
Realized capital gains and losses by asset type
Realized capital gains and losses	20	(4)	(6)	(4)
Limited partnership interests
Realized capital gains and losses by asset type
Realized capital gains and losses	(11)	10	(8)	23
Derivatives
Realized capital gains and losses by asset type
Realized capital gains and losses	10	(4)	7	(9)
Other
Realized capital gains and losses by asset type
Realized capital gains and losses	$ (1)	$ 0	$ (1)	$ (2)