Investments - Realized Capital Gains and Losses by Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments [Abstract]
Impairment write-downs	$ (1)	$ (21)
Change in intent write-downs	0	(3)
Net OTTI losses recognized in earnings	(1)	(24)
Sales	(5)	28
Valuation of equity investments	(23)	0
Valuation and settlements of derivative instruments	(3)	(5)
Total realized capital gains and losses	$ (32)	$ (1)