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Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenue from External Customer [Line Items]
Earned premiums	$ 3,156,382	$ 2,926,823	$ 9,266,948	$ 8,537,585
Insurance
Revenue from External Customer [Line Items]
Earned premiums		2,564,490		7,447,828
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Earned premiums	2,773,009	2,564,490	8,144,300	7,447,828
Insurance | Other liability
Revenue from External Customer [Line Items]
Earned premiums	1,122,591	1,031,087	3,299,651	3,000,335
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Earned premiums	636,751	563,736	1,852,847	1,613,342
Insurance | Auto
Revenue from External Customer [Line Items]
Earned premiums	408,213	376,762	1,203,794	1,092,354
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Earned premiums	318,284	313,154	948,194	925,214
Insurance | Professional liability
Revenue from External Customer [Line Items]
Earned premiums	287,170	279,751	839,814	816,583
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Earned premiums		362,333		1,089,757
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Earned premiums	383,373	362,333	1,122,648	1,089,757
Reinsurance & Monoline Excess | Casualty
Revenue from External Customer [Line Items]
Earned premiums	189,767	188,284	556,437	585,208
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Earned premiums	74,339	65,663	212,965	196,525
Reinsurance & Monoline Excess | Property
Revenue from External Customer [Line Items]
Earned premiums	$ 119,267	$ 108,386	$ 353,246	$ 308,024

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