Fair Value Measurements - Fair Value and Significant Unobservable Inputs Used in Pricing Debt Securities (Details) - Recurring - Significant Unobservable Inputs (Level 3) $ in Millions	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 915	$ 523
Debt securities | Securities Not Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	3	0
Corporate and government debt securities | Securities Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 765	$ 385
Corporate and government debt securities | Securities Priced by the Company | Minimum | Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0060	0.0070
Corporate and government debt securities | Securities Priced by the Company | Maximum | Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.1570	0.0930
Corporate and government debt securities | Securities Priced by the Company | Weighted Average | Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0410	0.0280
Mortgage and other asset-backed securities | Securities Priced by the Company
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 147	$ 138
Mortgage and other asset-backed securities | Securities Priced by the Company | Minimum | Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0060	0.0060
Mortgage and other asset-backed securities | Securities Priced by the Company | Minimum | Weighting of credit spreads
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0320	0.0240
Mortgage and other asset-backed securities | Securities Priced by the Company | Maximum | Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0370	0.0370
Mortgage and other asset-backed securities | Securities Priced by the Company | Maximum | Weighting of credit spreads
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0650	0.0460
Mortgage and other asset-backed securities | Securities Priced by the Company | Weighted Average | Liquidity
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0080	0.0070
Mortgage and other asset-backed securities | Securities Priced by the Company | Weighted Average | Weighting of credit spreads
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Unobservable Adjustment	0.0460	0.0330