Fair Value of Assets and Liabilities - Significant unobservable inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Quantitative information about the significant unobservable inputs
Fair value	$ 23,261	$ 24,222
Significant unobservable inputs (Level 3)
Quantitative information about the significant unobservable inputs
Fair value	(284)	(292)
Assets at fair value	467	526
Stochastic cash flow model | Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts – Equity-indexed and forward starting options
Quantitative information about the significant unobservable inputs
Fair value	$ (250)	$ (246)
Stochastic cash flow model | Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts – Equity-indexed and forward starting options | Maximum
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	2.20%	2.20%
Stochastic cash flow model | Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts – Equity-indexed and forward starting options | Minimum
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	1.00%	1.00%
Stochastic cash flow model | Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts – Equity-indexed and forward starting options | Weighted average
Quantitative information about the significant unobservable inputs
Projected option cost (as a percent)	1.74%	1.75%
Recurring basis | Significant unobservable inputs (Level 3)
Quantitative information about the significant unobservable inputs
Fair value	$ 376	$ 439
Assets at fair value		517
Non-recurring basis | Significant unobservable inputs (Level 3)
Quantitative information about the significant unobservable inputs
Assets at fair value		9
Non-recurring basis | Significant unobservable inputs (Level 3) | Fixed income securities - non-binding broker quotes
Quantitative information about the significant unobservable inputs
Assets at fair value	$ 237	$ 296