Fair Value Measurements - Fair Value and Significant Unobservable Inputs Used in Pricing Debt Securities (Details) - Recurring - Significant Unobservable Inputs (Level 3) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 751	$ 523
Debt securities | Securities Not Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	2	0
Corporate and government debt securities | Securities Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 582	$ 385
Corporate and government debt securities | Securities Priced by the Company | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment, Liquidity	0.70%	0.70%
Corporate and government debt securities | Securities Priced by the Company | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment, Liquidity	23.30%	9.30%
Corporate and government debt securities | Securities Priced by the Company | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment, Liquidity	3.70%	2.80%
Mortgage and other asset-backed securities | Securities Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 167	$ 138
Mortgage and other asset-backed securities | Securities Priced by the Company | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment, Liquidity	0.60%	0.60%
Unobservable Adjustment, Weighting of credit spreads	3.50%	2.40%
Mortgage and other asset-backed securities | Securities Priced by the Company | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment, Liquidity	3.50%	3.70%
Unobservable Adjustment, Weighting of credit spreads	5.80%	4.60%
Mortgage and other asset-backed securities | Securities Priced by the Company | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment, Liquidity	1.20%	0.70%
Unobservable Adjustment, Weighting of credit spreads	4.40%	3.30%