Segment Reporting Disclosures (Summary Of Segment Earned Premiums By Product) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Segment Reporting Information [Line Items]
Earned premiums	$ 5,049,793		$ 4,712,060		$ 4,247,978
Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	4,144,073		3,783,939		3,314,033
Insurance [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	1,039,617		889,543		764,956
Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	814,587		701,867		628,878
Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	364,830		369,116		365,513
Insurance [Member] | Marine And Energy [Member]
Segment Reporting Information [Line Items]
Earned premiums	391,464		376,747		312,282
Insurance [Member] | Personal Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums	378,522		374,543		382,761
Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Earned premiums	294,418		288,398		273,954
Insurance [Member] | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Earned premiums	349,770		329,690		319,679
Insurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	510,865		454,035		266,010
Reinsurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	903,587		928,574		934,114
Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	201,486		233,195		321,178
Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Earned premiums	408,368		360,739		327,912
Reinsurance [Member] | Specialty [Member]
Segment Reporting Information [Line Items]
Earned premiums	293,733		334,640		285,024
Other [Member]
Segment Reporting Information [Line Items]
Earned premiums	$ 2,133	[1]	$ (453)	[2]	$ (169)	[3]

[1] Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment as well as any amortization of intangible assets and impairment of goodwill and intangible assets that are not allocated to a reportable segment.

[2] Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment as well as any amortization of intangible assets that is not allocated to a reportable segment.

[3] Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment as well as any amortization of intangible assets and impairment of goodwill and intangible assets that are not allocated to a reportable segment.