Supplemental Cash Flow Information - Activities resulting from management of proceeds (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Net change in proceeds managed
Net change in fixed income securities	$ 175	$ 129
Net change in short-term investments	(454)	(157)
Operating cash flow (used) provided	(279)	(28)
Net change in cash	0	1
Net change in proceeds managed	(279)	(27)
Change in Liabilities for Collateral [Roll Forward]
Liabilities for collateral, beginning of period	(1,124)	(1,129)
Liabilities for collateral, end of period	(1,403)	(1,156)
Operating cash flow provided (used)	$ 279	$ 27