Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 9,561,429	$ 8,106,031	$ 6,930,843
Insurance
Segment Reporting Information [Line Items]
Net premiums earned	8,369,062	7,077,708	6,067,669
Insurance | Other liability
Segment Reporting Information [Line Items]
Net premiums earned	3,206,846	2,673,098	2,269,458
Insurance | Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned	1,630,371	1,389,068	1,247,908
Insurance | Workers' compensation
Segment Reporting Information [Line Items]
Net premiums earned	1,197,811	1,131,283	1,127,487
Insurance | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	1,208,241	990,945	794,171
Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned	1,125,793	893,314	628,645
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	1,192,367	1,028,323	863,174
Reinsurance & Monoline Excess | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	764,793	643,193	521,559
Reinsurance & Monoline Excess | Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	217,017	201,187	171,522
Reinsurance & Monoline Excess | Property
Segment Reporting Information [Line Items]
Net premiums earned	$ 210,557	$ 183,943	$ 170,093