Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 40,840	$ 39,818
Fixed maturities, at fair value using the fair value option ("FVO Securities")	292	327
Equity securities, at fair value [1]	736	864
Derivative assets	47	(1)
Short-term investments	2,975	3,850
Total assets accounted for at fair value on a recurring basis	44,890	44,858
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(22)	8
Total liabilities accounted for at fair value on a recurring basis	$ (22)	8
Equity Security, FV-NI, Contractual Sale Restriction, Nature of Restriction and Circumstance for Lapse	Level 3 includes investments that have contractual sales restrictions that require consent to sell and are in place for the duration that the securities are held by the Company.
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	$ 29	(10)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(29)	10
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	17	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	10	4
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	(3)	(6)
Asset backed securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,499	3,320
Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,168	3,090
Fixed maturities, at fair value using the fair value option ("FVO Securities")	167	167
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,050	3,125
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	18,657	17,866
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	548	562
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	5,941	6,039
RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,473	4,287
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,504	1,529
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	0	18
Fixed maturities, at fair value using the fair value option ("FVO Securities")	0	0
Equity securities, at fair value [1]	344	333
Derivative assets	0	0
Short-term investments	442	1,400
Total assets accounted for at fair value on a recurring basis	786	1,751
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	0	18
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	38,407	37,563
Fixed maturities, at fair value using the fair value option ("FVO Securities")	125	160
Equity securities, at fair value [1]	334	473
Derivative assets	47	(1)
Short-term investments	2,507	2,425
Total assets accounted for at fair value on a recurring basis	41,420	40,620
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(22)	8
Total liabilities accounted for at fair value on a recurring basis	(22)	8
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	29	(10)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(29)	10
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	17	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	10	4
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	(3)	(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	3,483	3,320
Significant Observable Inputs (Level 2) | Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,984	2,977
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,809	2,898
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	16,734	16,005
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	548	562
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	5,941	6,039
Significant Observable Inputs (Level 2) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,404	4,251
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,504	1,511
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,433	2,237
Fixed maturities, at fair value using the fair value option ("FVO Securities")	167	167
Equity securities, at fair value [1]	58	58
Derivative assets	0	0
Short-term investments	26	25
Total assets accounted for at fair value on a recurring basis	2,684	2,487
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	16	0
Significant Unobservable Inputs (Level 3) | Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	184	113
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	241	227
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,923	1,861
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	69	36
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	$ 75	$ 75