Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Revenue from External Customer [Line Items]
Net premiums earned	$ 3,098,185	$ 2,846,415	$ 6,110,566	$ 5,610,762
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		2,484,569		4,883,338
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,728,784	2,484,569	5,371,291	4,883,338
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	1,104,332	1,001,987	2,177,060	1,969,247
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	619,988	538,797	1,216,096	1,049,606
Insurance | Auto
Revenue from External Customer [Line Items]
Net premiums earned	405,632	361,580	795,581	715,593
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	318,881	310,564	629,910	612,060
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	279,951	271,641	552,644	536,832
Reinsurance and Monoline Excess Segment
Revenue from External Customer [Line Items]
Net premiums earned		361,846		727,424
Reinsurance and Monoline Excess Segment | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	369,401	361,846	739,275	727,424
Reinsurance and Monoline Excess Segment | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	184,903	199,080	366,670	396,924
Reinsurance and Monoline Excess Segment | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	71,362	65,511	138,626	130,862
Reinsurance and Monoline Excess Segment | Property
Revenue from External Customer [Line Items]
Net premiums earned	$ 113,136	$ 97,255	$ 233,979	$ 199,638