Fair Value of Assets and Liabilities - Quantitative information about the significant unobservable inputs used in level 3 fair value measurements (Details) - Equity-indexed and forward starting options in life and annuity product contracts - Significant unobservable inputs (Level 3) $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Quantitative information about the significant unobservable inputs
Fair value	$ (239)	$ (252)
Projected option cost | Weighted Average
Quantitative information about the significant unobservable inputs
Measurement Input	0.0174	0.0174
Projected option cost | Maximum
Quantitative information about the significant unobservable inputs
Measurement Input	0.022	0.022
Projected option cost | Minimum
Quantitative information about the significant unobservable inputs
Measurement Input	0.010	0.010