v3.26.1
FAIR VALUE MEASUREMENTS - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading $ 25,185,430 $ 24,884,323
Short term investments 3,949,012 4,759,811
Equity investments 1,846,152 1,732,990
Other investments 5,083,169 4,574,214
Assumed and ceded (re)insurance contracts   (34)
Derivative assets 31,284 18,347
Derivative liabilities (34,267) (41,200)
Total other liabilities (2,983) (22,887)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 36,060,780 35,928,451
Fair Value, Inputs, Level 1, 2 and 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,926,920 1,798,715
Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 9,308,212 10,641,503
Short term investments 8,815 959
Equity investments 1,846,152 1,732,990
Other investments 0 0
Assumed and ceded (re)insurance contracts   0
Derivative assets 19,808 7,400
Derivative liabilities (9,281) (30,477)
Total other liabilities   (23,077)
Total other assets 10,527  
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 11,173,706 12,352,375
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 15,565,754 14,088,705
Short term investments 3,940,197 4,758,852
Equity investments 0 0
Other investments 1,903,295 1,770,720
Assumed and ceded (re)insurance contracts   0
Derivative assets 11,476 10,947
Derivative liabilities (24,986) (10,723)
Total other liabilities (13,510)  
Total other assets   224
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 21,395,736 20,618,501
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 311,464 154,115
Short term investments 0 0
Equity investments 0 0
Other investments 23,625 27,995
Assumed and ceded (re)insurance contracts   (34)
Derivative assets 0 0
Derivative liabilities 0 0
Total other liabilities 0 (34)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 335,089 182,076
U.S. treasuries    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 9,308,212 10,641,503
U.S. treasuries | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 9,308,212 10,641,503
U.S. treasuries | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
U.S. treasuries | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 9,916,723 8,528,828
Corporate | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Corporate | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 9,697,026 8,414,572
Corporate | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 219,697 114,256
Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 2,690,648 2,606,882
Residential mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Residential mortgage-backed | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 2,690,648 2,606,882
Residential mortgage-backed | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 1,688,500 1,606,790
Asset-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Asset-backed | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 1,596,733 1,566,931
Asset-backed | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 91,767 39,859
Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 683,983 691,912
Non-U.S. government | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Non-U.S. government | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 683,983 691,912
Non-U.S. government | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 560,030 486,817
Agencies | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Agencies | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 560,030 486,817
Agencies | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 337,334 321,591
Commercial mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Commercial mortgage-backed | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 337,334 321,591
Commercial mortgage-backed | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Fixed income exchange traded funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 1,603,299 1,582,811
Fixed income exchange traded funds | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 1,603,299 1,582,811
Fixed income exchange traded funds | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 0 0
Fixed income exchange traded funds | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 0 0
Equity exchange traded funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 102,273  
Equity exchange traded funds | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 102,273  
Equity exchange traded funds | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 0  
Equity exchange traded funds | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 0  
Common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 140,580 150,179
Common stock | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 140,580 150,179
Common stock | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 0 0
Common stock | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 0 0
Catastrophe bonds | Fair Value, Inputs, Level 1, 2 and 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,772,044 1,613,710
Catastrophe bonds | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Catastrophe bonds | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,772,044 1,613,710
Catastrophe bonds | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Direct private equity investments | Fair Value, Inputs, Level 1, 2 and 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 154,876 185,005
Direct private equity investments | Quoted Prices in Active Markets for Identical  Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Direct private equity investments | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 131,251 157,010
Direct private equity investments | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 23,625 27,995
Fund investments | Fair Value Measured at Net Asset Value Per Share    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments $ 3,156,249 $ 2,775,499