Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 40,205	$ 42,148
Fixed maturities, FVO	8	11
Equity securities, at fair value	1,155	1,657
Derivative assets	10	12
Short-term investments	2,505	2,921
Total assets accounted for at fair value on a recurring basis	43,883	46,749
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(66)	(78)
Contingent consideration	0	(22)
Total liabilities accounted for at fair value on a recurring basis	(66)	(100)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	2	11
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	3	(1)
Equity
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(15)
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	8
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	19	(2)
Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		1
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(88)	(60)
Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,348	1,476
Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,989	2,183
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,302	4,338
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	16,798	17,396
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,063	1,123
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,497	9,498
RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,086	4,869
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,122	1,265
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	95	330
Fixed maturities, FVO	0	0
Equity securities, at fair value	822	1,401
Derivative assets	0	0
Short-term investments	1,649	1,028
Total assets accounted for at fair value on a recurring basis	2,566	2,759
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	95	330
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	38,791	40,404
Fixed maturities, FVO	8	11
Equity securities, at fair value	264	183
Derivative assets	10	12
Short-term investments	842	1,878
Total assets accounted for at fair value on a recurring basis	39,915	42,488
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(66)	(63)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(66)	(63)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	2	11
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	3	(1)
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	8
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	19	(2)
Significant Observable Inputs (Level 2) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		1
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(88)	(60)
Significant Observable Inputs (Level 2) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,329	1,461
Significant Observable Inputs (Level 2) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,906	2,088
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,284	4,329
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	16,089	16,664
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,060	1,120
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,497	9,498
Significant Observable Inputs (Level 2) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,599	4,309
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,027	935
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,319	1,414
Fixed maturities, FVO	0	0
Equity securities, at fair value	69	73
Derivative assets	0	0
Short-term investments	14	15
Total assets accounted for at fair value on a recurring basis	1,402	1,502
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	(15)
Contingent consideration	0	(22)
Total liabilities accounted for at fair value on a recurring basis	0	(37)
Significant Unobservable Inputs (Level 3) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Equity
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(15)
Significant Unobservable Inputs (Level 3) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	19	15
Significant Unobservable Inputs (Level 3) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	83	95
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	18	9
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	709	732
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3	3
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Significant Unobservable Inputs (Level 3) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	487	560
Significant Unobservable Inputs (Level 3) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Fair Value Measured at Net Asset Value Per Share [Member]
Liabilities accounted for at fair value on a recurring basis
Deposit Assets	$ 40	$ 38