Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 40,233	$ 42,847
Fixed maturities, at fair value using the fair value option ("FVO")	288	160
Equity securities, at fair value	1,910	2,094
Derivative assets	8	7
Short-term investments	3,937	3,697
Total assets accounted for at fair value on a recurring basis	46,376	48,805
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(22)	(49)
Total liabilities accounted for at fair value on a recurring basis	(22)	(49)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	2	2
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(2)	(4)
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	9	6
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	6	0
Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	(1)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(30)	(45)
Commodity Contract
Assets accounted for at fair value on a recurring basis
Derivative assets	(4)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(4)
Asset backed securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	942	1,135
Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,971	3,025
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,722	4,119
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	17,618	18,707
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	784	910
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	7,594	8,257
RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,936	3,643
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,666	3,051
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	829	882
Fixed maturities, at fair value using the fair value option ("FVO")	0	0
Equity securities, at fair value	1,343	1,453
Derivative assets	0	0
Short-term investments	2,222	1,627
Total assets accounted for at fair value on a recurring basis	4,394	3,962
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	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) | Commodity Contract
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
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	829	882
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	37,115	39,561
Fixed maturities, at fair value using the fair value option ("FVO")	114	0
Equity securities, at fair value	510	577
Derivative assets	12	6
Short-term investments	1,683	1,990
Total assets accounted for at fair value on a recurring basis	39,434	42,134
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(26)	(48)
Total liabilities accounted for at fair value on a recurring basis	(26)	(48)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	2	2
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(2)	(4)
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	9	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	6	1
Significant Observable Inputs (Level 2) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	(1)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(30)	(45)
Significant Observable Inputs (Level 2) | Commodity Contract
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
Significant Observable Inputs (Level 2) | Asset backed securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	923	1,135
Significant Observable Inputs (Level 2) | Collateralized Loan Obligations [Member]
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,757	2,768
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,486	3,923
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	16,103	17,089
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	780	905
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	7,594	8,257
Significant Observable Inputs (Level 2) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,635	3,315
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,837	2,169
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,289	2,404
Fixed maturities, at fair value using the fair value option ("FVO")	174	160
Equity securities, at fair value	57	64
Derivative assets	(4)	1
Short-term investments	32	80
Total assets accounted for at fair value on a recurring basis	2,548	2,709
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	4	(1)
Total liabilities accounted for at fair value on a recurring basis	4	(1)
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	1
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	(1)
Significant Unobservable Inputs (Level 3) | Interest rate 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) | Commodity Contract
Assets accounted for at fair value on a recurring basis
Derivative assets	(4)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(4)
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	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	214	257
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	236	196
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,515	1,618
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4	5
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	301	328
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	$ 69	$ 65