v3.25.1
Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Investments:    
Short-term and fixed maturity investments $ 6,503 $ 6,170
Funds held 2,357 2,451
Equity investments [1] 759 803
Other investments, at fair value [1] 4,206 4,188
Total Investments, excluding funds held by reinsured companies and equity method investments 13,825 13,612
Reinsurance balances recoverable on paid and unpaid losses 194 179
Derivative assets 12 5
Losses and loss adjustment expenses, at fair value 996 997
Derivative liabilities 8 7
Funds held by reinsurance companies, at amortized cost 2,315 2,528
Derivatives qualifying as hedging    
Investments:    
Derivative assets 1 2
Derivative liabilities 5  
Derivatives not qualifying as hedging    
Investments:    
Derivative assets 11 3
Derivative liabilities 3 7
U.S. government and agency    
Investments:    
Short-term and fixed maturity investments 579 420
U.K. government    
Investments:    
Short-term and fixed maturity investments 53 44
Other government    
Investments:    
Short-term and fixed maturity investments 364 359
Corporate    
Investments:    
Short-term and fixed maturity investments 3,427 3,261
Municipal    
Investments:    
Short-term and fixed maturity investments 105 109
Residential mortgage-backed    
Investments:    
Short-term and fixed maturity investments 444 421
Commercial mortgage-backed    
Investments:    
Short-term and fixed maturity investments 742 784
Asset-backed    
Investments:    
Short-term and fixed maturity investments 789 772
Equity investments | Privately held    
Investments:    
Equity investments 455 460
Equity investments | Publicly traded    
Investments:    
Equity investments 153 176
Exchange-traded funds    
Investments:    
Equity investments 135 151
Warrants and other    
Investments:    
Equity investments 16 16
Private equity funds    
Investments:    
Other investments, at fair value 1,949 1,926
Private credit funds    
Investments:    
Other investments, at fair value 915 864
Hedge funds    
Investments:    
Other investments, at fair value 386 410
Fixed income funds    
Investments:    
Other investments, at fair value 375 369
Real estate fund    
Investments:    
Other investments, at fair value 427 401
CLO equity funds    
Investments:    
Other investments, at fair value 124 162
CLO equities    
Investments:    
Other investments, at fair value 25 52
Equity funds    
Investments:    
Other investments, at fair value 5 4
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Investments:    
Short-term and fixed maturity investments 0 0
Funds held 70 70
Equity investments 278 317
Other investments, at fair value 0 0
Total Investments, excluding funds held by reinsured companies and equity method investments 348 387
Reinsurance balances recoverable on paid and unpaid losses 0 0
Derivative assets 0 0
Losses and loss adjustment expenses, at fair value 0 0
Derivative liabilities 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives qualifying as hedging    
Investments:    
Derivative assets 0 0
Derivative liabilities 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives not qualifying as hedging    
Investments:    
Derivative assets 0 0
Derivative liabilities 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. government and agency    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.K. government    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other government    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential mortgage-backed    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial mortgage-backed    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed    
Investments:    
Short-term and fixed maturity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments | Privately held    
Investments:    
Equity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments | Publicly traded    
Investments:    
Equity investments 143 166
Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange-traded funds    
Investments:    
Equity investments 135 151
Quoted Prices in Active Markets for Identical Assets (Level 1) | Warrants and other    
Investments:    
Equity investments 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Private equity funds    
Investments:    
Other investments, at fair value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Private credit funds    
Investments:    
Other investments, at fair value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Hedge funds    
Investments:    
Other investments, at fair value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income funds    
Investments:    
Other investments, at fair value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate fund    
Investments:    
Other investments, at fair value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | CLO equity funds    
Investments:    
Other investments, at fair value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | CLO equities    
Investments:    
Other investments, at fair value 0 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity funds    
Investments:    
Other investments, at fair value 0 0
Significant Other Observable Inputs (Level 2)    
Investments:    
Short-term and fixed maturity investments 6,460 6,123
Funds held 2,209 2,305
Equity investments 9 9
Other investments, at fair value 421 424
Total Investments, excluding funds held by reinsured companies and equity method investments 9,099 8,861
Reinsurance balances recoverable on paid and unpaid losses 0 0
Derivative assets 12 5
Losses and loss adjustment expenses, at fair value 0 0
Derivative liabilities 8  
Significant Other Observable Inputs (Level 2) | Derivatives qualifying as hedging    
Investments:    
Derivative assets 1 2
Derivative liabilities 5  
Significant Other Observable Inputs (Level 2) | Derivatives not qualifying as hedging    
Investments:    
Derivative assets 11 3
Derivative liabilities 3 7
Significant Other Observable Inputs (Level 2) | U.S. government and agency    
Investments:    
Short-term and fixed maturity investments 579 420
Significant Other Observable Inputs (Level 2) | U.K. government    
Investments:    
Short-term and fixed maturity investments 53 44
Significant Other Observable Inputs (Level 2) | Other government    
Investments:    
Short-term and fixed maturity investments 364 359
Significant Other Observable Inputs (Level 2) | Corporate    
Investments:    
Short-term and fixed maturity investments 3,414 3,244
Significant Other Observable Inputs (Level 2) | Municipal    
Investments:    
Short-term and fixed maturity investments 105 109
Significant Other Observable Inputs (Level 2) | Residential mortgage-backed    
Investments:    
Short-term and fixed maturity investments 444 421
Significant Other Observable Inputs (Level 2) | Commercial mortgage-backed    
Investments:    
Short-term and fixed maturity investments 742 784
Significant Other Observable Inputs (Level 2) | Asset-backed    
Investments:    
Short-term and fixed maturity investments 759 742
Significant Other Observable Inputs (Level 2) | Equity investments | Privately held    
Investments:    
Equity investments 0 0
Significant Other Observable Inputs (Level 2) | Equity investments | Publicly traded    
Investments:    
Equity investments 9 9
Significant Other Observable Inputs (Level 2) | Exchange-traded funds    
Investments:    
Equity investments 0 0
Significant Other Observable Inputs (Level 2) | Warrants and other    
Investments:    
Equity investments 0 0
Significant Other Observable Inputs (Level 2) | Private equity funds    
Investments:    
Other investments, at fair value 0 0
Significant Other Observable Inputs (Level 2) | Private credit funds    
Investments:    
Other investments, at fair value 386 363
Significant Other Observable Inputs (Level 2) | Hedge funds    
Investments:    
Other investments, at fair value 0 0
Significant Other Observable Inputs (Level 2) | Fixed income funds    
Investments:    
Other investments, at fair value 5 5
Significant Other Observable Inputs (Level 2) | Real estate fund    
Investments:    
Other investments, at fair value 0 0
Significant Other Observable Inputs (Level 2) | CLO equity funds    
Investments:    
Other investments, at fair value 0 0
Significant Other Observable Inputs (Level 2) | CLO equities    
Investments:    
Other investments, at fair value 25 52
Significant Other Observable Inputs (Level 2) | Equity funds    
Investments:    
Other investments, at fair value 5 4
Significant Unobservable Inputs (Level 3)    
Investments:    
Short-term and fixed maturity investments 43 47
Funds held 4 5
Equity investments 401 406
Other investments, at fair value 0 0
Total Investments, excluding funds held by reinsured companies and equity method investments 448 458
Reinsurance balances recoverable on paid and unpaid losses 194 179
Derivative assets 0 0
Losses and loss adjustment expenses, at fair value 996 997
Derivative liabilities 0  
Significant Unobservable Inputs (Level 3) | Derivatives qualifying as hedging    
Investments:    
Derivative assets 0 0
Derivative liabilities 0  
Significant Unobservable Inputs (Level 3) | Derivatives not qualifying as hedging    
Investments:    
Derivative assets 0 0
Derivative liabilities 0 0
Significant Unobservable Inputs (Level 3) | U.S. government and agency    
Investments:    
Short-term and fixed maturity investments 0 0
Significant Unobservable Inputs (Level 3) | U.K. government    
Investments:    
Short-term and fixed maturity investments 0 0
Significant Unobservable Inputs (Level 3) | Other government    
Investments:    
Short-term and fixed maturity investments 0 0
Significant Unobservable Inputs (Level 3) | Corporate    
Investments:    
Short-term and fixed maturity investments 13 17
Significant Unobservable Inputs (Level 3) | Municipal    
Investments:    
Short-term and fixed maturity investments 0 0
Significant Unobservable Inputs (Level 3) | Residential mortgage-backed    
Investments:    
Short-term and fixed maturity investments 0 0
Significant Unobservable Inputs (Level 3) | Commercial mortgage-backed    
Investments:    
Short-term and fixed maturity investments 0 0
Significant Unobservable Inputs (Level 3) | Asset-backed    
Investments:    
Short-term and fixed maturity investments 30 30
Significant Unobservable Inputs (Level 3) | Equity investments | Privately held    
Investments:    
Equity investments 384 389
Significant Unobservable Inputs (Level 3) | Equity investments | Publicly traded    
Investments:    
Equity investments 1 1
Significant Unobservable Inputs (Level 3) | Exchange-traded funds    
Investments:    
Equity investments 0 0
Significant Unobservable Inputs (Level 3) | Warrants and other    
Investments:    
Equity investments 16 16
Significant Unobservable Inputs (Level 3) | Private equity funds    
Investments:    
Other investments, at fair value 0 0
Significant Unobservable Inputs (Level 3) | Private credit funds    
Investments:    
Other investments, at fair value 0 0
Significant Unobservable Inputs (Level 3) | Hedge funds    
Investments:    
Other investments, at fair value 0 0
Significant Unobservable Inputs (Level 3) | Fixed income funds    
Investments:    
Other investments, at fair value 0 0
Significant Unobservable Inputs (Level 3) | Real estate fund    
Investments:    
Other investments, at fair value 0 0
Significant Unobservable Inputs (Level 3) | CLO equity funds    
Investments:    
Other investments, at fair value 0 0
Significant Unobservable Inputs (Level 3) | CLO equities    
Investments:    
Other investments, at fair value 0 0
Significant Unobservable Inputs (Level 3) | Equity funds    
Investments:    
Other investments, at fair value 0 0
Measured Using NAV as Practical Expedient    
Investments:    
Funds held 74 71
Equity investments 71 71
Other investments, at fair value 3,785 3,764
Total Investments, excluding funds held by reinsured companies and equity method investments 3,930 3,906
Measured Using NAV as Practical Expedient | Equity investments | Privately held    
Investments:    
Equity investments 71 71
Measured Using NAV as Practical Expedient | Warrants and other    
Investments:    
Equity investments 0  
Measured Using NAV as Practical Expedient | Private equity funds    
Investments:    
Other investments, at fair value 1,949 1,926
Measured Using NAV as Practical Expedient | Private credit funds    
Investments:    
Other investments, at fair value 529 501
Measured Using NAV as Practical Expedient | Hedge funds    
Investments:    
Other investments, at fair value 386 410
Measured Using NAV as Practical Expedient | Fixed income funds    
Investments:    
Other investments, at fair value 370 364
Measured Using NAV as Practical Expedient | Real estate fund    
Investments:    
Other investments, at fair value 427 401
Measured Using NAV as Practical Expedient | CLO equity funds    
Investments:    
Other investments, at fair value 124 162
Measured Using NAV as Practical Expedient | CLO equities    
Investments:    
Other investments, at fair value 0 0
Measured Using NAV as Practical Expedient | Equity funds    
Investments:    
Other investments, at fair value $ 0 $ 0
[1] See Note 17 for additional information regarding related party transactions.