RESERVE FOR CLAIMS AND CLAIM EXPENSES - Schedule of Reserve for Claims and Claim Expenses by Segment (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 3,983,325	$ 3,821,053
Additional Case Reserves	2,005,242	2,176,884
IBNR	9,966,598	9,894,636
Gross reserve for claims and claim expenses	15,955,165	15,892,573	$ 15,662,955
Property | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	1,944,257	1,956,688
Additional Case Reserves	1,791,160	2,008,891
IBNR	2,647,186	3,570,253
Gross reserve for claims and claim expenses	6,382,603	7,535,832
Casualty and Specialty | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	2,039,068	1,864,365
Additional Case Reserves	214,082	167,993
IBNR	7,319,412	6,324,383
Gross reserve for claims and claim expenses	$ 9,572,562	$ 8,356,741