Investments - Change in unrealized net capital gains and losses (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in Unrealized Gain (Loss) Recognized in Accumulated Other Comprehensive Income (Loss)
Change in unrealized net capital gains and losses	$ (1,326)
Amount recognized for:
Insurance reserves	315
DAC and DSI	124
Amounts recognized	439
Deferred income taxes	189
Decrease in unrealized net capital gains and losses, after-tax	(698)
Fixed income securities
Change in Unrealized Gain (Loss) Recognized in Accumulated Other Comprehensive Income (Loss)
Change in unrealized net capital gains and losses	(1,326)
Equity securities
Change in Unrealized Gain (Loss) Recognized in Accumulated Other Comprehensive Income (Loss)
Change in unrealized net capital gains and losses	0
Derivatives
Change in Unrealized Gain (Loss) Recognized in Accumulated Other Comprehensive Income (Loss)
Change in unrealized net capital gains and losses	(2)
Retained income
Amount recognized for:
Cumulative effect of change in accounting principle		$ 1,088	$ 0
Retained income | Accounting Standards Update 2016-01
Amount recognized for:
Cumulative effect of change in accounting principle		$ 1,160
EMA limited partnerships
Change in Unrealized Gain (Loss) Recognized in Accumulated Other Comprehensive Income (Loss)
Change in unrealized net capital gains and losses	$ 2