v3.26.3
FAIR VALUE MEASUREMENTS - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading $ 25,203,247 $ 24,884,323
Short term investments 4,009,923 4,759,811
Equity investments 1,767,229 1,732,990
Other investments 5,047,962 4,574,214
Assumed and ceded (re)insurance contracts   (34)
Derivative assets 19,385 18,347
Derivative liabilities (33,954) (41,200)
Total other liabilities (14,569) (22,887)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 36,013,792 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 1,780,844 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 9,037,223 10,641,503
Short term investments 18,193 959
Equity investments 1,767,229 1,732,990
Other investments 0 0
Assumed and ceded (re)insurance contracts   0
Derivative assets 9,576 7,400
Derivative liabilities (6,985) (30,477)
Total other liabilities   (23,077)
Total other assets 2,591  
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 10,825,236 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 15,782,219 14,088,705
Short term investments 3,991,730 4,758,852
Equity investments 0 0
Other investments 1,756,282 1,770,720
Assumed and ceded (re)insurance contracts   0
Derivative assets 9,809 10,947
Derivative liabilities (26,969) (10,723)
Total other liabilities (17,160)  
Total other assets   224
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 21,513,071 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 383,805 154,115
Short term investments 0 0
Equity investments 0 0
Other investments 24,562 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 408,367 182,076
Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 9,807,924 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 9,560,977 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 246,947 114,256
U.S. Treasuries    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 9,037,223 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 9,037,223 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
Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 3,449,802 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 3,449,802 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 1,357,970 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 1,221,112 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 136,858 39,859
Agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 641,239 486,817
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 641,239 486,817
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
Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 580,896 691,912
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 580,896 691,912
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
Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 328,193 321,591
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 328,193 321,591
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 1,547,207 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 1,547,207 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
Common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 129,409 150,179
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 129,409 150,179
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
Equity exchange traded funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity investments 90,613  
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 90,613  
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  
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 1,625,250 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 1,625,250 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 155,594 185,005
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 131,032 157,010
Direct private equity investments | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 24,562 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 $ 3,267,118 $ 2,775,499