Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,926,823	$ 2,641,914	$ 8,537,585	$ 7,686,073
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		2,291,917		6,650,027
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,564,490	2,291,917	7,447,828	6,650,027
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	1,031,087	916,394	3,000,335	2,660,850
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	563,736	472,805	1,613,342	1,330,173
Insurance | Auto
Revenue from External Customer [Line Items]
Net premiums earned	376,762	327,507	1,092,354	929,201
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	313,154	300,694	925,214	905,405
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	279,751	274,517	816,583	824,398
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		349,997		1,036,046
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	362,333	349,997	1,089,757	1,036,046
Reinsurance & Monoline Excess | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	188,284	201,795	585,208	618,855
Reinsurance & Monoline Excess | Property
Revenue from External Customer [Line Items]
Net premiums earned	108,386	87,838	308,024	239,338
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	$ 65,663	$ 60,364	$ 196,525	$ 177,853