v3.25.4
FAIR VALUE MEASUREMENTS - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
Dec. 31, 2025
Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading $ 24,884,323,000 $ 23,562,514,000
Short term investments 4,759,811,000 4,531,655,000
Total equity investments 1,732,990,000 117,756,000
Other investments 4,574,214,000 4,324,761,000
Assumed and ceded (re)insurance contracts (34,000) (405,000)
Derivative assets 18,347,000 34,538,000
Derivative liabilities (41,200,000) (75,878,000)
Other liabilities (22,887,000) (41,745,000)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 35,928,451,000 32,494,941,000
U.S. treasuries    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 10,641,503,000 11,001,893,000
Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 8,528,828,000 7,862,423,000
Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 2,606,882,000 1,707,056,000
Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 1,606,790,000 1,422,393,000
Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 691,912,000 618,809,000
Agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 486,817,000 623,489,000
Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 321,591,000 326,451,000
Fixed income exchange traded funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 1,582,811,000  
Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 150,179,000 117,756,000
Fair Value, Inputs, Level 1, 2 and 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,798,715,000 2,196,262,000
Fair Value, Inputs, Level 1, 2 and 3 | Catastrophe bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,613,710,000 1,984,396,000
Fair Value, Inputs, Level 1, 2 and 3 | Direct private equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 185,005,000 211,866,000
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 10,641,503,000 11,001,893,000
Short term investments 959,000 99,343,000
Total equity investments 1,732,990,000  
Other investments 0 0
Assumed and ceded (re)insurance contracts 0 0
Derivative assets 7,400,000 10,614,000
Derivative liabilities (30,477,000) (7,872,000)
Other liabilities (23,077,000)  
Other assets   2,742,000
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 12,352,375,000 11,221,734,000
Quoted Prices in Active Markets for Identical  Assets (Level 1) | U.S. treasuries    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 10,641,503,000 11,001,893,000
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fixed income exchange traded funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 1,582,811,000  
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 150,179,000 117,756,000
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Catastrophe bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Direct private equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 14,088,705,000 12,560,621,000
Short term investments 4,758,852,000 4,432,312,000
Total equity investments 0  
Other investments 1,770,720,000 2,152,485,000
Assumed and ceded (re)insurance contracts 0 0
Derivative assets 10,947,000 23,924,000
Derivative liabilities (10,723,000) (68,006,000)
Other liabilities   (44,082,000)
Other assets 224,000  
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 20,618,501,000 19,101,336,000
Significant Other Observable Inputs (Level 2) | U.S. treasuries    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Significant Other Observable Inputs (Level 2) | Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 8,414,572,000 7,862,423,000
Significant Other Observable Inputs (Level 2) | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 2,606,882,000 1,707,056,000
Significant Other Observable Inputs (Level 2) | Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 1,566,931,000 1,422,393,000
Significant Other Observable Inputs (Level 2) | Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 691,912,000 618,809,000
Significant Other Observable Inputs (Level 2) | Agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 486,817,000 623,489,000
Significant Other Observable Inputs (Level 2) | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 321,591,000 326,451,000
Significant Other Observable Inputs (Level 2) | Fixed income exchange traded funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 0  
Significant Other Observable Inputs (Level 2) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 0 0
Significant Other Observable Inputs (Level 2) | Catastrophe bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 1,613,710,000 1,984,396,000
Significant Other Observable Inputs (Level 2) | Direct private equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 157,010,000 168,089,000
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 154,115,000 0
Short term investments 0 0
Total equity investments 0  
Other investments 27,995,000 43,777,000
Assumed and ceded (re)insurance contracts (34,000) (405,000)
Derivative assets 0 0
Derivative liabilities 0 0
Other liabilities (34,000) (405,000)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs 182,076,000 43,372,000
Significant Unobservable Inputs (Level 3) | U.S. treasuries    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Significant Unobservable Inputs (Level 3) | Corporate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 114,256,000 0
Significant Unobservable Inputs (Level 3) | Residential mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Significant Unobservable Inputs (Level 3) | Asset-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 39,859,000 0
Significant Unobservable Inputs (Level 3) | Non-U.S. government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Significant Unobservable Inputs (Level 3) | Agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Significant Unobservable Inputs (Level 3) | Commercial mortgage-backed    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed maturity investments trading 0 0
Significant Unobservable Inputs (Level 3) | Fixed income exchange traded funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 0  
Significant Unobservable Inputs (Level 3) | Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total equity investments 0 0
Significant Unobservable Inputs (Level 3) | Catastrophe bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 0 0
Significant Unobservable Inputs (Level 3) | Direct private equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments 27,995,000 43,777,000
Significant Unobservable Inputs (Level 3) | Other assets | Assumed and ceded (re)insurance contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other assets   2,000,000.0
Significant Unobservable Inputs (Level 3) | Other Liabilities | Assumed and ceded (re)insurance contracts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other liabilities   (2,400,000)
Fair Value Measured at Net Asset Value Per Share | Fund investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Other investments $ 2,775,499,000 $ 2,128,499,000