Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	$ 763,486	$ 676,145	$ 556,168
Reserve for Losses and Loss Expenses	17,011,223	15,390,888	13,784,430
Unearned Premiums	5,297,654	4,847,160	4,073,191
Net Premiums Earned	9,561,429	8,106,031	6,930,843
Net Investment Income	779,185	671,618	583,821
Loss and Loss Expenses	5,861,750	4,953,960	4,468,706
Amortization of Deferred Policy Acquisition Cost	1,038,903	961,628	904,955
Other Operating Costs and Expenses	1,922,602	1,637,642	1,485,437
Net Premiums Written	10,004,070	8,862,867	7,262,437
Operating Segments | Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	651,257	566,718	467,871
Reserve for Losses and Loss Expenses	13,786,112	12,379,395	10,977,674
Unearned Premiums	4,779,214	4,348,171	3,660,758
Net Premiums Earned	8,369,062	7,077,708	6,067,669
Net Investment Income	550,084	468,821	375,554
Loss and Loss Expenses	5,130,909	4,326,403	3,939,759
Amortization of Deferred Policy Acquisition Cost	935,469	830,199	734,062
Other Operating Costs and Expenses	1,430,456	1,202,192	1,137,002
Net Premiums Written	8,784,146	7,743,814	6,347,101
Operating Segments | Reinsurance & Monoline Excess
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	112,229	109,427	88,297
Reserve for Losses and Loss Expenses	3,225,111	3,011,493	2,806,756
Unearned Premiums	518,440	498,989	412,433
Net Premiums Earned	1,192,367	1,028,323	863,174
Net Investment Income	194,272	175,324	146,029
Loss and Loss Expenses	730,841	627,557	528,947
Amortization of Deferred Policy Acquisition Cost	103,434	131,429	170,893
Other Operating Costs and Expenses	235,836	174,098	103,775
Net Premiums Written	1,219,924	1,119,053	915,336
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	34,829	27,473	62,238
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Costs and Expenses	256,310	261,352	244,660
Net Premiums Written	$ 0	$ 0	$ 0