Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 10,400,687	$ 9,561,429	$ 8,106,031
Insurance
Segment Reporting Information [Line Items]
Net premiums earned	9,130,324	8,369,062	7,077,708
Insurance | Other liability
Segment Reporting Information [Line Items]
Net premiums earned	3,613,900	3,206,846	2,673,098
Insurance | Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned	1,897,070	1,630,371	1,389,068
Insurance | Workers' compensation
Segment Reporting Information [Line Items]
Net premiums earned	1,212,294	1,197,811	1,131,283
Insurance | Auto
Segment Reporting Information [Line Items]
Net premiums earned	1,313,210	1,208,241	990,945
Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned	1,093,850	1,125,793	893,314
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	1,270,363	1,192,367	1,028,323
Reinsurance & Monoline Excess | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	770,920	764,793	643,193
Reinsurance & Monoline Excess | Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	241,126	217,017	201,187
Reinsurance & Monoline Excess | Property
Segment Reporting Information [Line Items]
Net premiums earned	$ 258,317	$ 210,557	$ 183,943