Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,748,921	$ 1,676,810	$ 5,117,253	$ 4,916,507
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,493,854		4,396,071
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	1,531,093	1,493,854	4,481,092	4,396,071
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	561,595	527,253	1,653,189	1,523,534
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	326,019	310,423	917,466	905,365
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	271,802	321,872	852,101	979,059
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	203,047	191,136	583,024	560,340
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	168,630	143,170	475,312	427,773
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		182,956		520,436
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	217,828	182,956	636,161	520,436
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	130,186	106,532	383,375	292,471
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	43,577	40,639	126,800	119,020
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 44,065	$ 35,785	$ 125,986	$ 108,945