Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	$ 556,168	$ 517,364	$ 497,629
Reserve for Losses and Loss Expenses	13,784,430	12,583,249	11,966,448
Unearned Premiums	4,073,191	3,656,507	3,359,991
Net Premiums Earned	6,930,843	6,633,288	6,371,505
Net Investment Income	583,821	645,614	674,235
Loss and Loss Expenses	4,468,706	4,131,116	3,974,702
Amortization of Deferred Policy Acquisition Cost	904,955	1,001,611	915,246
Other Operating Costs and Expenses	1,485,437	1,360,471	1,467,975
Net Premiums Written	7,262,437	6,863,499	6,433,227
Operating Segments | Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	467,871	438,082	432,568
Reserve for Losses and Loss Expenses	10,977,674	9,836,950	9,278,729
Unearned Premiums	3,660,758	3,304,152	3,081,433
Net Premiums Earned	6,067,669	5,919,819	5,702,073
Net Investment Income	375,554	429,405	433,490
Loss and Loss Expenses	3,939,759	3,692,551	3,566,358
Amortization of Deferred Policy Acquisition Cost	734,062	840,333	788,508
Other Operating Costs and Expenses	1,137,002	1,049,328	1,136,269
Net Premiums Written	6,347,101	6,086,009	5,791,905
Operating Segments | Reinsurance & Monoline Excess
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	88,297	79,282	65,061
Reserve for Losses and Loss Expenses	2,806,756	2,746,299	2,687,719
Unearned Premiums	412,433	352,355	278,558
Net Premiums Earned	863,174	713,469	669,432
Net Investment Income	146,029	164,082	179,534
Loss and Loss Expenses	528,947	438,565	408,344
Amortization of Deferred Policy Acquisition Cost	170,893	161,278	126,738
Other Operating Costs and Expenses	103,775	88,520	112,885
Net Premiums Written	915,336	777,490	641,322
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	62,238	52,127	61,211
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Costs and Expenses	244,660	222,623	218,821
Net Premiums Written	$ 0	$ 0	$ 0