Supplemental Cash Flow Information - Activities resulting from management of proceeds (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Net change in proceeds managed
Net change in fixed income securities	$ 122	$ 10
Net change in short-term investments	(663)	(7)
Operating cash flow (used) provided	(541)	3
Change in Liabilities for Collateral [Roll Forward]
Liabilities for collateral, beginning of period	(1,124)	(1,129)
Liabilities for collateral, end of period	(1,665)	(1,126)
Operating cash flow provided (used)	$ 541	$ (3)