Segment Reporting Disclosures (Summary Of Segment Earned Premiums By Product) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Earned premiums	$ 4,712,060		$ 4,247,978		$ 3,865,870
Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	3,783,939		3,314,033		3,028,844
Insurance [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	889,543		764,956		675,199
Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	701,867		628,878		600,607
Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	369,116		365,513		357,320
Insurance [Member] | Marine And Energy [Member]
Segment Reporting Information [Line Items]
Earned premiums	376,747		312,282		277,608
Insurance [Member] | Personal Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums	374,543		382,761		374,175
Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Earned premiums	288,398		273,954		263,783
Insurance [Member] | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Earned premiums	329,690		319,679		301,126
Insurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	454,035		266,010		179,026
Reinsurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	928,574		934,114		836,264
Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	233,195		321,178		288,771
Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Earned premiums	417,601		351,457		327,383
Reinsurance [Member] | Auto [Member]
Segment Reporting Information [Line Items]
Earned premiums	12,422		28,700		65,363
Reinsurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	265,356		232,779		154,747
Other [Member]
Segment Reporting Information [Line Items]
Earned premiums	$ (453)	[1]	$ (169)	[2]	$ 762	[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.