Fair Value of Assets and Liabilities - Level 3 Fair Value Measurement (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Quantitative information about the significant unobservable inputs
Fixed income securities	$ 22,401	$ 23,261
Significant unobservable inputs (Level 3)
Quantitative information about the significant unobservable inputs
Assets, fair value disclosure	469	467
Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Fair value	$ (230)	$ (250)
Weighted average projected option cost (as a percent)	1.74%	1.74%
Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Minimum | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	1.00%	1.00%
Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Maximum | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	2.20%	2.20%
Significant unobservable inputs (Level 3) | Fixed Income Securities Valued Based on Nonbinding Broker Quotes
Quantitative information about the significant unobservable inputs
Assets, fair value disclosure	$ 235	$ 237
Recurring basis | Significant unobservable inputs (Level 3)
Quantitative information about the significant unobservable inputs
Fixed income securities	$ 369	$ 376