Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 8,106,031	$ 6,930,843	$ 6,633,288
Insurance
Segment Reporting Information [Line Items]
Net premiums earned	7,077,708	6,067,669	5,919,819
Insurance | Other liability
Segment Reporting Information [Line Items]
Net premiums earned	2,639,601	2,237,285	2,063,401
Insurance | Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned	1,389,068	1,247,908	1,223,902
Insurance | Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned	1,131,283	1,127,487	1,301,980
Insurance | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	990,945	794,171	750,051
Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned	926,811	660,818	580,485
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	1,028,323	863,174	713,469
Reinsurance & Monoline Excess | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	643,193	521,559	405,063
Reinsurance & Monoline Excess | Monoline Excess
Segment Reporting Information [Line Items]
Net premiums earned	201,187	171,522	160,071
Reinsurance & Monoline Excess | Property
Segment Reporting Information [Line Items]
Net premiums earned	$ 183,943	$ 170,093	$ 148,335