Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 11,548,485	$ 10,400,687	$ 9,561,429
Insurance
Segment Reporting Information [Line Items]
Net premiums earned	10,086,308	9,007,376	8,171,828
Insurance \| Other liability
Segment Reporting Information [Line Items]
Net premiums earned	4,068,662	3,605,298	3,188,399
Insurance \| Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned	2,201,661	1,825,027	1,562,122
Insurance \| Workers' compensation
Segment Reporting Information [Line Items]
Net premiums earned	1,237,888	1,212,294	1,197,810
Insurance \| Auto
Segment Reporting Information [Line Items]
Net premiums earned	1,481,569	1,270,907	1,097,704
Insurance \| Professional liability
Segment Reporting Information [Line Items]
Net premiums earned	1,096,528	1,093,850	1,125,793
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	1,462,177	1,393,311	1,389,601
Reinsurance & Monoline Excess \| Casualty
Segment Reporting Information [Line Items]
Net premiums earned	771,329	821,826	893,777
Reinsurance & Monoline Excess \| Monoline Excess (3)
Segment Reporting Information [Line Items]
Net premiums earned	266,552	241,126	217,017
Reinsurance & Monoline Excess \| Property (2)
Segment Reporting Information [Line Items]
Net premiums earned	$ 424,296	$ 330,359	$ 278,807