Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,676,915	$ 1,646,841	$ 3,368,333	$ 3,239,697
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,475,184		2,902,218
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	1,465,044	1,475,184	2,949,999	2,902,218
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	544,465	505,621	1,091,594	996,282
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	295,968	305,852	591,446	594,943
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	278,699	330,510	580,299	657,186
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	190,335	188,278	379,978	369,203
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	155,577	144,923	306,682	284,604
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		171,657		337,479
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	211,871	171,657	418,334	337,479
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	130,459	95,109	253,190	185,939
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	41,062	39,400	83,224	78,381
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 40,350	$ 37,148	$ 81,920	$ 73,159