Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 42,200	$ 42,148
Fixed maturities, FVO	1	11
Equity securities, at fair value	756	1,657
Derivative assets	12	12
Short-term investments	3,668	2,921
Total assets accounted for at fair value on a recurring basis	46,637	46,749
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(70)	(78)
Contingent consideration		(22)
Total liabilities accounted for at fair value on a recurring basis	(70)	(100)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	7	11
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(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	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	16	(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	(86)	(60)
Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,416	1,476
Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,187	2,183
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,211	4,338
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	18,563	17,396
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	972	1,123
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,394	9,498
RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,895	4,869
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,562	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	450	330
Fixed maturities, FVO	0	0
Equity securities, at fair value	320	1,401
Derivative assets	0	0
Short-term investments	3,125	1,028
Total assets accounted for at fair value on a recurring basis	3,895	2,759
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Contingent consideration		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
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	450	330
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	40,020	40,404
Fixed maturities, FVO	1	11
Equity securities, at fair value	370	183
Derivative assets	12	12
Short-term investments	529	1,878
Total assets accounted for at fair value on a recurring basis	40,932	42,488
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(70)	(63)
Contingent consideration		0
Total liabilities accounted for at fair value on a recurring basis	(70)	(63)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	7	11
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(1)
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	16	(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	(86)	(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	1,393	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	2,088	2,088
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,191	4,329
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	17,454	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	972	1,120
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,394	9,498
Significant Observable Inputs (Level 2) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,416	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	1,112	935
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,730	1,414
Fixed maturities, FVO	0	0
Equity securities, at fair value	66	73
Derivative assets	0	0
Short-term investments	14	15
Total assets accounted for at fair value on a recurring basis	1,810	1,502
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	(15)
Contingent consideration		(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
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	23	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	99	95
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	20	9
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,109	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	0	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	479	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	$ 48	$ 38