Investments - Realized capital gains and losses by asset type (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Realized capital gains and losses by asset type
Realized capital gains and losses	$ 445	$ (90)	$ 30
Fixed income securities
Realized capital gains and losses by asset type
Realized capital gains and losses	94	(91)	212
Equity securities
Realized capital gains and losses by asset type
Realized capital gains and losses	255	23	(50)
Mortgage loans
Realized capital gains and losses by asset type
Realized capital gains and losses	1	0	6
Limited partnership interests
Realized capital gains and losses by asset type
Realized capital gains and losses	132	(21)	(93)
Derivatives
Realized capital gains and losses by asset type
Realized capital gains and losses	(46)	3	(21)
Other
Realized capital gains and losses by asset type
Realized capital gains and losses	$ 9	$ (4)	$ (24)