RESERVE FOR CLAIMS AND CLAIM EXPENSES - Claims and Claim Expense Reserves by Segment (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 3,934,746	$ 3,821,053
Additional Case Reserves	2,258,808	2,176,884
IBNR	9,803,272	9,894,636
Gross reserve for claims and claim expenses	15,996,826	15,892,573	$ 13,510,304
Property | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	2,017,567	1,956,688
Additional Case Reserves	2,069,014	2,008,891
IBNR	3,100,878	3,570,253
Gross reserve for claims and claim expenses	7,187,459	7,535,832
Casualty and Specialty | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	1,917,179	1,864,365
Additional Case Reserves	189,794	167,993
IBNR	6,702,394	6,324,383
Gross reserve for claims and claim expenses	$ 8,809,367	$ 8,356,741