SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	$ 473,569	$ 465,593	$ 431,439
Reserve for losses and loss expenses	15,168,863	14,653,094	13,926,766
Unearned premiums	4,361,447	4,090,676	3,685,886
Net premiums earned	5,160,326	4,709,850	4,371,309
Net investment income	418,829	454,301	349,601
Net losses and loss expenses	3,242,410	3,008,783	3,281,252
Acquisition costs	1,022,017	921,834	929,517
Other operating expenses	680,343	663,304	579,790
Net premiums written	5,263,056	4,926,624	4,336,409
General and administrative expenses	680,343	663,304	579,790
Operating Segments
Supplementary Insurance Information [Line Items]
General and administrative expenses	550,289	536,834	477,968
Operating Segments | Insurance
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	249,407	212,681	176,749
Reserve for losses and loss expenses	8,381,593	7,803,529	7,310,498
Unearned premiums	3,077,628	2,716,240	2,295,763
Net premiums earned	3,134,155	2,651,339	2,299,038
Net investment income	0	0	0
Net losses and loss expenses	1,785,854	1,514,998	1,697,014
Acquisition costs	577,838	484,344	461,533
Other operating expenses	443,704	429,282	378,839
Net premiums written	3,377,906	2,894,885	2,357,501
General and administrative expenses	443,704	429,282	378,839
Operating Segments | Reinsurance
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	224,162	252,912	254,690
Reserve for losses and loss expenses	6,787,270	6,849,565	6,616,268
Unearned premiums	1,283,819	1,374,436	1,390,123
Net premiums earned	2,026,171	2,058,511	2,072,271
Net investment income	0	0	0
Net losses and loss expenses	1,456,556	1,493,785	1,584,238
Acquisition costs	444,179	437,490	467,984
Other operating expenses	106,585	107,552	99,129
Net premiums written	1,885,150	2,031,739	1,978,908
General and administrative expenses	106,585	107,552	99,129
Corporate
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	0	0	0
Reserve for losses and loss expenses	0	0	0
Unearned premiums	0	0	0
Net premiums earned	0	0	0
Net investment income	418,829	454,301	349,601
Net losses and loss expenses	0	0	0
Acquisition costs	0	0	0
Other operating expenses	130,054	126,470	101,822
Net premiums written	0	0	0
General and administrative expenses	$ 130,054	$ 126,470	$ 101,822