FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	$ 15,888,564	$ 14,351,402
Short term investments	6,373,969	4,669,272
Equity investments trading	93,058	625,058
Other investments	3,091,686	2,494,954
Derivative assets	25,307	44,421
Derivative liabilities	(14,989)	(7,581)
Other assets	557	3,499
Other liabilities	(1,032)	(5,331)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	15,888,564	14,351,402
Short term investments	6,373,969	4,669,272
Equity investments trading	93,058	625,058
Other investments	3,091,686	2,494,954
Assumed and ceded (re)insurance contracts	(475)	(1,832)
Derivative assets	25,307	44,400
Derivative liabilities	(14,989)	(7,560)
Other assets	9,843	35,008
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	25,457,120	22,175,694
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	1,850,339	1,408,248
Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	8,592,242	7,180,129
Short term investments	94,474	0
Equity investments trading	93,058	625,058
Other investments	0	0
Assumed and ceded (re)insurance contracts	0	0
Derivative assets	370	387
Derivative liabilities	(2,113)	(2,008)
Other liabilities	(1,743)	(1,621)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	8,778,031	7,803,566
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	7,296,322	7,171,273
Short term investments	6,279,495	4,669,272
Equity investments trading	0	0
Other investments	1,679,184	1,241,468
Assumed and ceded (re)insurance contracts	0	0
Derivative assets	24,937	44,013
Derivative liabilities	(12,876)	(5,552)
Other assets	12,061	38,461
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	15,267,062	13,120,474
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	0	0
Short term investments	0	0
Equity investments trading	0	0
Other investments	171,155	166,780
Assumed and ceded (re)insurance contracts	(475)	(1,832)
Derivative assets	0	0
Derivative liabilities	0	0
Other liabilities	(475)	(1,832)
Total assets and (liabilities) measured at fair value on a recurring basis using Level 3 inputs	170,680	164,948
Significant Unobservable Inputs (Level 3) | Other assets | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assumed and ceded (re)insurance contracts	(600)	(3,500)
Significant Unobservable Inputs (Level 3) | Other liabilities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assumed and ceded (re)insurance contracts	(1,000)	(5,300)
U.S. treasuries | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	8,592,242	7,180,129
U.S. treasuries | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	8,592,242	7,180,129
U.S. treasuries | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	0	0
Corporate | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	4,371,407	4,390,568
Corporate | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	4,371,407	4,390,568
Corporate | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	0	0
Agencies | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	476,476	395,149
Agencies | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	476,476	395,149
Agencies | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	401,743	383,838
Non-U.S. government | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	401,743	383,838
Non-U.S. government | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	0	0
Residential mortgage-backed | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	788,256	710,429
Residential mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	788,256	710,429
Residential mortgage-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	0	0
Commercial mortgage-backed | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	209,661	213,987
Commercial mortgage-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	209,661	213,987
Commercial mortgage-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	0	0
Asset-backed | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	1,048,779	1,077,302
Asset-backed | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	1,048,779	1,077,302
Asset-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity investments trading	0	0
Catastrophe bonds | Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	1,679,184	1,241,468
Catastrophe bonds | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	1,679,184	1,241,468
Catastrophe bonds | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Term loans | Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	100,000	100,000
Term loans | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0
Term loans | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0
Term loans | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	100,000	100,000
Direct private equity investments | Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	71,155	66,780
Direct private equity investments | Quoted Prices in Active Markets for Identical  Assets (Level 1) | Fair Value, Measurements, Recurring
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, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	0	0
Direct private equity investments | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	71,155	66,780
Fund investments | Fair Value Measured at Net Asset Value Per Share | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	$ 1,241,347	$ 1,086,706