Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,646,841	$ 1,581,053	$ 3,239,697	$ 3,148,461
Insurance
Revenue from External Customer [Line Items]
Net premiums earned	1,475,184	1,415,579	2,902,218	2,806,868
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	505,621	474,721	996,282	938,889
Insurance | Workers’ compensation
Revenue from External Customer [Line Items]
Net premiums earned	330,510	336,996	657,186	661,974
Insurance | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	305,852	288,320	594,943	583,347
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	188,278	175,812	369,203	351,010
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	144,923	139,730	284,604	271,648
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned	171,657	165,474	337,479	341,593
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	37,148	38,204	73,159	82,705
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	95,109	86,841	185,939	177,411
Reinsurance & Monoline Excess | Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned	$ 39,400	$ 40,429	$ 78,381	$ 81,477