Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,552,727	$ 2,357,158	$ 5,044,159	$ 4,606,244
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		2,070,157		4,032,991
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,246,394	2,070,157	4,428,269	4,032,991
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	893,629	789,363	1,750,778	1,538,754
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	462,554	401,768	893,057	776,980
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	318,421	301,741	629,843	583,975
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	299,149	299,645	604,710	585,067
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	272,641	277,640	549,881	548,215
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		287,001		573,253
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	306,333	287,001	615,890	573,253
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	188,860	190,659	382,590	374,781
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	58,845	54,611	117,489	106,507
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 58,628	$ 41,731	$ 115,811	$ 91,965