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) - Fair Value, Recurring [Member] - Significant Unobservable Inputs (Level 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	$ (4)	$ (4)	$ (33)	$ (6)
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	0	(2)	12	(14)
Contingent Consideration
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	0	0	12	(6)
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	24	(3)	(5)	6
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	0	0	0	0
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	0	0	0	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	(2)	(9)	(2)
Equity Securities [Member] | 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	0	0	0	0
Freestanding Derivatives, net | Other liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	(2)	0	(8)
Freestanding Derivatives, net | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0	0	(1)
Freestanding Derivatives, net | Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	(2)	0	(8)
Freestanding Derivatives, net | Equity | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0	0	0
Freestanding 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	0	(1)
Freestanding Derivatives, net | Other Comprehensive Income (Loss) [Member] | Other liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0	0	0
Freestanding Derivatives, net | Other Comprehensive Income (Loss) [Member] | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0	0	0
Freestanding Derivatives, net | Other Comprehensive Income (Loss) [Member] | Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0	0	0
Freestanding Derivatives, net | Other Comprehensive Income (Loss) [Member] | Equity | Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	0	0	0	0
Freestanding Derivatives, net | Other Comprehensive Income (Loss) [Member] | 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	0	0
Total Fixed Maturities, AFS | Securities available-for-sale and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	(4)	(2)	(24)	(3)
Total Fixed Maturities, AFS | Securities available-for-sale and other | 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	24	(3)	(5)	6
Total Fixed Maturities, AFS | Collateralized loan obligations ("CLOs") | Securities available-for-sale and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0	0	0
Total Fixed Maturities, AFS | Collateralized loan obligations ("CLOs") | Securities available-for-sale and other | 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	1	0	(1)	1
Total Fixed Maturities, AFS | CMBS | Securities available-for-sale and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0	0	0
Total Fixed Maturities, AFS | CMBS | Securities available-for-sale and other | 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	2	0	2	0
Total Fixed Maturities, AFS | Corporate | Securities available-for-sale and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	(1)	(2)	(21)	(3)
Total Fixed Maturities, AFS | Corporate | Securities available-for-sale and other | 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	17	0	1	8
Total Fixed Maturities, AFS | Foreign Government/Government Agencies | Securities available-for-sale and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0	0	0
Total Fixed Maturities, AFS | Foreign Government/Government Agencies | Securities available-for-sale and other | 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	0	0	0	1
Total Fixed Maturities, AFS | Municipal | Securities available-for-sale and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	(3)	0	(3)	0
Total Fixed Maturities, AFS | Municipal | Securities available-for-sale and other | 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	2	0	2	0
Total Fixed Maturities, AFS | RMBS | Securities available-for-sale and other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	0	0	0	0
Total Fixed Maturities, AFS | RMBS | Securities available-for-sale and other | 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	$ 2	$ (3)	$ (9)	$ (4)