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	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Assets	$ (4)	$ (1)	$ (6)	$ (1)
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	(3)	(4)	6	(20)
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	0	0
Significant Unobservable Inputs (Level 3) | Freestanding 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	(1)	(1)	(1)
Significant Unobservable Inputs (Level 3) | Freestanding Derivatives, net | Liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	(2)	0	(8)	0
Significant Unobservable Inputs (Level 3) | 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	(1)	0	(1)
Significant Unobservable Inputs (Level 3) | Freestanding Derivatives, net | Equity | Liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Derivatives	(2)	0	(8)	0
Significant Unobservable Inputs (Level 3) | 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	(1)	0
Significant Unobservable Inputs (Level 3) | Freestanding Derivatives, net | 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, Derivatives	0	0	0	0
Significant Unobservable Inputs (Level 3) | Freestanding Derivatives, net | Other Comprehensive Income (Loss) [Member] | Liability [Member]
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
Significant Unobservable Inputs (Level 3) | 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
Significant Unobservable Inputs (Level 3) | Freestanding Derivatives, net | Other Comprehensive Income (Loss) [Member] | Equity | Liability [Member]
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
Significant Unobservable Inputs (Level 3) | 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
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | 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	(2)	0	(2)	0
Significant Unobservable Inputs (Level 3) | Securities available-for-sale and other | 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
Significant Unobservable Inputs (Level 3) | 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	(2)	0	(3)	0
Significant Unobservable Inputs (Level 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	(3)	(4)	6	(20)
Significant Unobservable Inputs (Level 3) | Total Fixed Maturities, AFS | 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
Significant Unobservable Inputs (Level 3) | Total Fixed Maturities, AFS | 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	0	0	1	0
Significant Unobservable Inputs (Level 3) | 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
Significant Unobservable Inputs (Level 3) | 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	0	1	0	1
Significant Unobservable Inputs (Level 3) | 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	(2)	0	(3)	0
Significant Unobservable Inputs (Level 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	0	(2)	8	(11)
Significant Unobservable Inputs (Level 3) | Total Fixed Maturities, AFS | Foreign Govt./Govt. 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
Significant Unobservable Inputs (Level 3) | Total Fixed Maturities, AFS | Foreign Govt./Govt. 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	1	0
Significant Unobservable Inputs (Level 3) | 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
Significant Unobservable Inputs (Level 3) | 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	(3)	(3)	(4)	(10)
Fair Value, Recurring [Member] | 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	(2)	(1)	(14)	(3)
Fair Value, Recurring [Member] | 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	0	(1)	(6)	(3)
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) | Liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	(2)	(1)	(14)	(3)
Fair Value, Recurring [Member] | 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	$ 0	$ 0	$ 0	$ 0