Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	$ 676,145	$ 556,168	$ 517,364
Reserve for Losses and Loss Expenses	15,390,888	13,784,430	12,583,249
Unearned Premiums	4,847,160	4,073,191	3,656,507
Net Premiums Earned	8,106,031	6,930,843	6,633,288
Net Investment Income	671,618	583,821	645,614
Loss and Loss Expenses	4,953,960	4,468,706	4,131,116
Amortization of Deferred Policy Acquisition Cost	961,628	904,955	1,001,611
Other Operating Costs and Expenses	1,637,642	1,485,437	1,360,471
Net Premiums Written	8,862,867	7,262,437	6,863,499
Operating Segments | Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	566,718	467,871	438,082
Reserve for Losses and Loss Expenses	12,379,395	10,977,674	9,836,950
Unearned Premiums	4,348,171	3,660,758	3,304,152
Net Premiums Earned	7,077,708	6,067,669	5,919,819
Net Investment Income	468,821	375,554	429,405
Loss and Loss Expenses	4,326,403	3,939,759	3,692,551
Amortization of Deferred Policy Acquisition Cost	830,199	734,062	840,333
Other Operating Costs and Expenses	1,202,192	1,137,002	1,049,328
Net Premiums Written	7,743,814	6,347,101	6,086,009
Operating Segments | Reinsurance & Monoline Excess
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	109,427	88,297	79,282
Reserve for Losses and Loss Expenses	3,011,493	2,806,756	2,746,299
Unearned Premiums	498,989	412,433	352,355
Net Premiums Earned	1,028,323	863,174	713,469
Net Investment Income	175,324	146,029	164,082
Loss and Loss Expenses	627,557	528,947	438,565
Amortization of Deferred Policy Acquisition Cost	131,429	170,893	161,278
Other Operating Costs and Expenses	174,098	103,775	88,520
Net Premiums Written	1,119,053	915,336	777,490
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	27,473	62,238	52,127
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Costs and Expenses	261,352	244,660	222,623
Net Premiums Written	$ 0	$ 0	$ 0