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Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,846,415	$ 2,552,727	$ 5,610,762	$ 5,044,159
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		2,215,186		4,358,110
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,484,569	2,215,186	4,883,338	4,358,110
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	1,001,987	890,984	1,969,247	1,744,456
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	538,797	444,834	1,049,606	857,368
Insurance | Auto
Revenue from External Customer [Line Items]
Net premiums earned	361,580	307,578	715,593	601,694
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	310,564	299,149	612,060	604,710
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	271,641	272,641	536,832	549,882
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		337,541		686,049
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	361,846	337,541	727,424	686,049
Reinsurance & Monoline Excess | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	199,080	202,348	396,924	417,060
Reinsurance & Monoline Excess | Property
Revenue from External Customer [Line Items]
Net premiums earned	97,255	76,348	199,638	151,500
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	$ 65,511	$ 58,845	$ 130,862	$ 117,489

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 235
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17(Column E))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480149/944-235-S99-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479655/944-605-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(c))
-SubTopic 220
-Topic 942
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(1))
-SubTopic 220
-Topic 944
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1

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