Fair Value of Assets and Liabilities - Level 3 Fair Value Measurement (Details) - Equity-indexed and forward starting options in life and annuity product contracts - Significant unobservable inputs (Level 3) $ in Millions	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Quantitative information about the significant unobservable inputs
Fair value	$ (228)	$ (250)
Projected option cost | Minimum
Quantitative information about the significant unobservable inputs
Embedded derivative liability, measurement input	0.010	0.010
Projected option cost | Maximum
Quantitative information about the significant unobservable inputs
Embedded derivative liability, measurement input	0.022	0.022
Projected option cost | Weighted average
Quantitative information about the significant unobservable inputs
Embedded derivative liability, measurement input	0.0174	0.0174