Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 6,633,288	$ 6,371,505	$ 6,311,419
Insurance
Segment Reporting Information [Line Items]
Net premiums earned	5,919,819	5,702,073	5,549,403
Insurance | Other liability
Segment Reporting Information [Line Items]
Net premiums earned	2,063,401	1,912,071	1,843,826
Insurance | Workers' compensation
Segment Reporting Information [Line Items]
Net premiums earned	1,301,980	1,327,206	1,324,801
Insurance | Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned	1,223,902	1,184,447	1,184,465
Insurance | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	750,051	722,236	650,441
Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned	580,485	556,113	545,870
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	713,469	669,432	762,016
Reinsurance & Monoline Excess | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	405,063	362,886	377,650
Reinsurance & Monoline Excess | Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	160,071	162,908	157,039
Reinsurance & Monoline Excess | Property
Segment Reporting Information [Line Items]
Net premiums earned	$ 148,335	$ 143,638	$ 227,327