Fair Value of Assets and Liabilities - Quantitative information about the significant unobservable inputs used in level 3 fair value measurements (Details) - Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options - Stochastic cash flow model - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Quantitative information about the significant unobservable inputs
Liabilities, fair value	$ (232)	$ (252)
Weighted average projected option cost (as a percent)	1.74%	1.74%
Minimum
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	1.00%	1.00%
Maximum
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	2.20%	2.20%