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FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)	12 Months Ended
FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 4,574,214,000	$ 4,324,761,000
Fixed maturity investments trading	24,884,323,000	23,562,514,000
Debt	2,329,201,000	1,886,689,000
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments trading	8,528,828,000	7,862,423,000
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments trading	1,606,790,000	1,422,393,000
Other Investments \| Net investment income
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains included in net investment income	106,100,000	243,800,000	$ 137,300,000
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	1,770,720,000	2,152,485,000
Fixed maturity investments trading	14,088,705,000	12,560,621,000
Debt	2,200,000,000	1,900,000,000
Fair Value	2,300,000,000	1,900,000,000
Significant Other Observable Inputs (Level 2) \| Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	157,010,000	168,089,000
Significant Other Observable Inputs (Level 2) \| Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments trading	8,414,572,000	7,862,423,000
Significant Other Observable Inputs (Level 2) \| Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments trading	1,566,931,000	1,422,393,000
Significant Other Observable Inputs (Level 2) \| Valuation Technique, Quoted Prices \| Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	157,000,000.0	168,100,000
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	27,995,000	43,777,000
Fixed maturity investments trading	154,115,000	0
Debt	99,200,000	0
Fair Value	100,200,000	0
Significant Unobservable Inputs (Level 3) \| Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	27,995,000	43,777,000
Significant Unobservable Inputs (Level 3) \| Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments trading	114,256,000	0
Significant Unobservable Inputs (Level 3) \| Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturity investments trading	39,859,000	0
Significant Unobservable Inputs (Level 3) \| Internal valuation model \| Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 27,995,000	$ 43,777,000