Investments - Realized capital gains and losses by asset type (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	$ (25)	$ 81	$ (159)	$ 215
Fixed income securities
Realized capital gains and losses by asset type
Realized capital gains and losses	(80)	32	(123)	37
Equity securities
Realized capital gains and losses by asset type
Realized capital gains and losses	74	19	(19)	125
Mortgage loans
Realized capital gains and losses by asset type
Realized capital gains and losses	2	0	2	0
Limited partnership interests
Realized capital gains and losses by asset type
Realized capital gains and losses	(43)	31	(33)	71
Derivatives
Realized capital gains and losses by asset type
Realized capital gains and losses	23	(8)	15	(23)
Other
Realized capital gains and losses by asset type
Realized capital gains and losses	$ (1)	$ 7	$ (1)	$ 5