Investments - Realized capital gains and losses by asset type (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	$ 176	$ 103	$ 17	$ 318
Fixed income securities
Realized capital gains and losses by asset type
Realized capital gains and losses	(30)	41	(153)	78
Equity securities
Realized capital gains and losses by asset type
Realized capital gains and losses	223	57	204	182
Mortgage loans
Realized capital gains and losses by asset type
Realized capital gains and losses	0	1	2	1
Limited partnership interests
Realized capital gains and losses by asset type
Realized capital gains and losses	(23)	21	(56)	92
Derivatives
Realized capital gains and losses by asset type
Realized capital gains and losses	5	(17)	20	(40)
Other
Realized capital gains and losses by asset type
Realized capital gains and losses	$ 1	$ 0	$ 0	$ 5