Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 37,444	$ 36,231
Fixed maturities, at fair value using the fair value option ("FVO")	326	333
Equity securities, at fair value	1,301	1,801
Derivative assets	50	34
Short-term investments	4,060	3,859
Total assets accounted for at fair value on a recurring basis	43,181	42,258
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(31)	13
Total liabilities accounted for at fair value on a recurring basis	(31)	13
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	3	2
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(26)	(2)
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	46	32
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	2	21
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	(7)	(6)
Asset backed securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,181	1,941
Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,013	2,941
Fixed maturities, at fair value using the fair value option ("FVO")	172	178
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,329	3,368
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	16,210	15,233
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	549	547
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	6,365	6,296
RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,737	3,708
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,060	2,197
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	1	0
Fixed maturities, at fair value using the fair value option ("FVO")	0	0
Equity securities, at fair value	750	1,261
Derivative assets	0	0
Short-term investments	2,498	1,429
Total assets accounted for at fair value on a recurring basis	3,249	2,690
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	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) | Foreign exchange 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) | 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 [Member]
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	1	0
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	35,325	34,180
Fixed maturities, at fair value using the fair value option ("FVO")	154	155
Equity securities, at fair value	491	479
Derivative assets	50	34
Short-term investments	1,375	2,237
Total assets accounted for at fair value on a recurring basis	37,395	37,085
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(31)	13
Total liabilities accounted for at fair value on a recurring basis	(31)	13
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	3	2
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(26)	(2)
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	46	32
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	2	21
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	(7)	(6)
Significant Observable Inputs (Level 2) | Asset backed securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,145	1,911
Significant Observable Inputs (Level 2) | Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,862	2,826
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,098	3,146
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	14,567	13,644
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	549	547
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	6,365	6,296
Significant Observable Inputs (Level 2) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,680	3,613
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,059	2,197
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,118	2,051
Fixed maturities, at fair value using the fair value option ("FVO")	172	178
Equity securities, at fair value	60	61
Derivative assets	0	0
Short-term investments	187	193
Total assets accounted for at fair value on a recurring basis	2,537	2,483
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Total liabilities accounted for at fair value on a recurring basis	0	0
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) | Foreign exchange 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) | 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	36	30
Significant Unobservable Inputs (Level 3) | Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	151	115
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	231	222
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,643	1,589
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	0
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	57	95
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