FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 3,167,941		$ 3,167,941		$ 2,494,954
Other liabilities	(3,686)		(3,686)		(5,331)
Debt	1,882,893		1,882,893		1,170,442
Long-term debt, fair value	1,800,000		1,800,000		1,100,000
Net Investment Income | Term loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net unrealized gains (losses) recognized in earnings	20,200	$ 137,400	100,600	$ 208,100
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	3,167,941		3,167,941		2,494,954
Other liabilities	(17,869)		(17,869)
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	166,391		166,391		166,780
Other liabilities	(545)		(545)		(1,832)
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	67,515		67,515		66,780
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Term loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	98,876		98,876		100,000
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring | Assumed and ceded (re)insurance contracts, group two
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	(545)		(545)		(1,832)
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring | Direct private equity investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	67,515		67,515		66,780
Significant Unobservable Inputs (Level 3) | Internal valuation model | Fair Value, Measurements, Recurring | Term loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	$ 98,900		$ 98,900		$ 100,000
U.S. Treasuries
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	4.80%		4.80%		4.30%
Corporate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	6.70%		6.70%		6.30%
Agencies
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	5.30%		5.30%		4.60%
Non-U.S. Government
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	5.10%		5.10%		4.70%
Residential Mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	5.90%		5.90%		5.40%
Weighted average life			8 years		8 years 7 months 6 days
Commercial Mortgage-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	8.70%		8.70%		7.40%
Weighted average life			2 years 2 months 12 days		3 years 2 months 12 days
Asset-backed
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	7.40%		7.40%		7.40%
Weighted average life			4 years 3 months 18 days		5 years 2 months 12 days
Short Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	5.40%		5.40%		4.00%
Other Short Term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Weighted average effective yield	5.30%		5.30%		4.20%
Private Credit Funds | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			5 years
Private Credit Funds | Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			10 years
Direct private equity investments | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			5 years
Direct private equity investments | Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liquidation period for fund assets			10 years