Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	$ 517,364	$ 497,629	$ 507,549
Reserve for Losses and Loss Expenses	12,583,249	11,966,448	11,670,408
Unearned Premiums	3,656,507	3,359,991	3,290,180
Net Premiums Earned	6,633,288	6,371,505	6,311,419
Net Investment Income	645,614	674,235	575,788
Loss and Loss Expenses	4,131,116	3,974,702	4,002,348
Amortization of Deferred Policy Acquisition Cost	1,001,611	915,246	1,111,489
Other Operating Costs and Expenses	1,360,471	1,467,975	1,325,443
Net Premiums Written	6,863,499	6,433,227	6,260,508
Operating Segments | Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	438,082	432,568	428,535
Reserve for Losses and Loss Expenses	9,836,950	9,278,729	8,858,926
Unearned Premiums	3,304,152	3,081,433	2,987,805
Net Premiums Earned	5,919,819	5,702,073	5,549,403
Net Investment Income	429,405	433,490	366,862
Loss and Loss Expenses	3,692,551	3,566,358	3,469,175
Amortization of Deferred Policy Acquisition Cost	840,333	788,508	908,746
Other Operating Costs and Expenses	1,049,328	1,136,269	1,001,463
Net Premiums Written	6,086,009	5,791,905	5,555,515
Operating Segments | Reinsurance & Monoline Excess
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Cost	79,282	65,061	79,014
Reserve for Losses and Loss Expenses	2,746,299	2,687,719	2,811,482
Unearned Premiums	352,355	278,558	302,375
Net Premiums Earned	713,469	669,432	762,016
Net Investment Income	164,082	179,534	160,462
Loss and Loss Expenses	438,565	408,344	533,173
Amortization of Deferred Policy Acquisition Cost	161,278	126,738	202,743
Other Operating Costs and Expenses	88,520	112,885	69,431
Net Premiums Written	777,490	641,322	704,993
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	52,127	61,211	48,464
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Costs and Expenses	222,623	218,821	254,549
Net Premiums Written	$ 0	$ 0	$ 0