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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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,357,158	$ 1,971,624	$ 4,606,244	$ 3,821,580
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,727,202		3,332,181
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,070,157	1,727,202	4,032,991	3,332,181
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	789,363	646,321	1,538,754	1,255,582
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	401,768	339,890	776,980	664,630
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	299,645	291,471	585,067	558,920
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	301,741	237,444	583,975	458,205
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	277,640	212,077	548,215	394,844
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		244,422		489,399
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	287,001	244,422	573,253	489,399
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	190,659	152,531	374,781	302,169
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	54,611	48,247	106,507	94,120
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 41,731	$ 43,644	$ 91,965	$ 93,110

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(1))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17(Column E))
-URI http://asc.fasb.org/extlink&oid=120401096&loc=d3e574992-122915

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(c))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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