Investments - Realized capital gains and losses (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	$ 49	$ (77)	$ 265
Fixed income securities
Realized capital gains and losses by asset type
Realized capital gains and losses	(6)	(59)	309
Mortgage loans
Realized capital gains and losses by asset type
Realized capital gains and losses	1	0	6
Equity securities
Realized capital gains and losses by asset type
Realized capital gains and losses	21	(22)	(23)
Limited partnership interests
Realized capital gains and losses by asset type
Realized capital gains and losses	46	(5)	(32)
Derivatives
Realized capital gains and losses by asset type
Realized capital gains and losses	(16)	8	13
Other
Realized capital gains and losses by asset type
Realized capital gains and losses	$ 3	$ 1	$ (8)