Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	$ 951,728	$ 861,609	$ 763,486
Reserve for Losses and Loss Expenses	20,368,030	18,739,652	17,011,223
Unearned Premiums	6,375,112	5,922,326	5,297,654
Net Premiums Earned	11,548,485	10,400,687	9,561,429
Net Investment Income	1,333,161	1,052,835	779,185
Loss and Loss Expenses	7,131,595	6,372,142	5,861,750
Amortization of Deferred Policy Acquisition Cost	1,219,849	1,038,975	1,038,903
Other Operating Costs and Expenses	2,382,457	2,324,961	1,922,602
Net Premiums Written	11,972,096	10,954,467	10,004,070
Operating Segments | Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	840,917	736,348	633,493
Reserve for Losses and Loss Expenses	16,887,821	15,298,372	13,655,613
Unearned Premiums	5,836,435	5,322,869	4,708,586
Net Premiums Earned	10,086,308	9,007,376	8,171,828
Net Investment Income	1,057,738	783,660	543,844
Loss and Loss Expenses	6,332,490	5,615,526	5,013,614
Amortization of Deferred Policy Acquisition Cost	1,081,170	897,908	881,567
Other Operating Costs and Expenses	1,825,758	1,684,514	1,418,374
Net Premiums Written	10,549,550	9,560,533	8,609,028
Operating Segments | Reinsurance & Monoline Excess
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	110,811	125,261	129,993
Reserve for Losses and Loss Expenses	3,480,209	3,441,280	3,355,610
Unearned Premiums	538,677	599,457	589,068
Net Premiums Earned	1,462,177	1,393,311	1,389,601
Net Investment Income	234,728	221,966	200,512
Loss and Loss Expenses	799,105	756,616	848,136
Amortization of Deferred Policy Acquisition Cost	138,679	141,067	157,336
Other Operating Costs and Expenses	292,526	268,309	247,918
Net Premiums Written	1,422,546	1,393,934	1,395,042
Corporate, other and eliminations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	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	40,695	47,209	34,829
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Costs and Expenses	264,173	372,138	256,310
Net Premiums Written	$ 0	$ 0	$ 0