Investments - Change in unrealized net capital gains and losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 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	$ (1,002)
Amount recognized for:
Insurance reserves	196
DAC and DSI	87
Amounts recognized	283
Deferred income taxes	154
Decrease in unrealized net capital gains and losses, after-tax	(565)
Fixed income securities
Change in Unrealized Gain (Loss) Recognized in Accumulated Other Comprehensive Income (Loss)
Change in unrealized net capital gains and losses	(1,002)
Equity securities
Change in Unrealized Gain (Loss) Recognized in Accumulated Other Comprehensive Income (Loss)
Change in unrealized net capital gains and losses	$ 0
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