Fair Value Measurements - Changes in Unrealized Gains (Losses) Included in Net Income for Financial Instruments Classified as Level 3 Still Held at Year End (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	$ (20)	$ 0
Other Comprehensive Income (Loss) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	10	28
Significant Unobservable Inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	(7)	(6)
Significant Unobservable Inputs (Level 3) | Contingent Consideration
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	(7)	(6)
Significant Unobservable Inputs (Level 3) | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	1	0
Significant Unobservable Inputs (Level 3) | Derivatives, net
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	(19)	1
Significant Unobservable Inputs (Level 3) | Derivatives, net | Equity derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	(18)	1
Significant Unobservable Inputs (Level 3) | Derivatives, net | Interest rate derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	(1)	0
Significant Unobservable Inputs (Level 3) | Other Comprehensive Income (Loss) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	0	0
Significant Unobservable Inputs (Level 3) | Other Comprehensive Income (Loss) [Member] | Contingent Consideration
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities
Significant Unobservable Inputs (Level 3) | Other Comprehensive Income (Loss) [Member] | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0
Significant Unobservable Inputs (Level 3) | Other Comprehensive Income (Loss) [Member] | Derivatives, net
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0
Significant Unobservable Inputs (Level 3) | Other Comprehensive Income (Loss) [Member] | Derivatives, net | Equity derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0
Significant Unobservable Inputs (Level 3) | Other Comprehensive Income (Loss) [Member] | Derivatives, net | Interest rate derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	(2)	(1)
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	(1)
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	(2)	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Debt Security, Government, Non-US [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Residential mortgage-backed securities (RMBS)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Other Comprehensive Income (Loss) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	10	28
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Other Comprehensive Income (Loss) [Member] | ABS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	1
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Other Comprehensive Income (Loss) [Member] | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	1	28
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Other Comprehensive Income (Loss) [Member] | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	15	(42)
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Other Comprehensive Income (Loss) [Member] | Debt Security, Government, Non-US [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	1	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Other Comprehensive Income (Loss) [Member] | States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	24
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | Total fixed maturities | Other Comprehensive Income (Loss) [Member] | Residential mortgage-backed securities (RMBS)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	$ (7)	$ 17