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)	$ (43)
Change in intent write-downs	0	(16)
Net OTTI losses recognized in earnings	(1)	(59)
Sales	(42)	208
Valuation of equity investments	(83)	0
Valuation and settlements of derivative instruments	(8)	(15)
Total realized capital gains and losses	$ (134)	$ 134