Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 21, 2019
Loss Contingencies [Line Items]
Maximum remaining term of noncancelable operating leases	16 years
Rental expense	$ 52,900	$ 44,600	$ 40,200
Goodwill, Impairment Loss	91,910	0	(18,700)
Impairment of Intangible Assets (Excluding Goodwill)	107,300
Impairment of Goodwill and Intangible Assets	199,198	0	18,723
Markel Ventures Operations [Member]
Loss Contingencies [Line Items]
Goodwill, Impairment Loss	0
Impairment of Goodwill and Intangible Assets	14,904		18,723
Industrial Products [Member]
Loss Contingencies [Line Items]
Impairment of Goodwill and Intangible Assets	14,900
Industrial Products [Member] | Markel Ventures Operations [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	33,500
Intangible Assets, Net (Including Goodwill)	0
Markel CATCo IM [Member]
Loss Contingencies [Line Items]
Accrued incentive and retention compensation	52,900	34,900
Compensation Expense	12,300	$ 38,100	$ 33,200
Accrued and unpaid incentive and retention compensation	19,200
Goodwill, Impairment Loss	91,900
Impairment of Intangible Assets (Excluding Goodwill)	87,100
Impairment of Goodwill and Intangible Assets	179,000
Intangible Assets, Net (Including Goodwill)	$ 0
Markel CATCo IM [Member] | Markel CATCo IM [Member]
Loss Contingencies [Line Items]
AssetAcquisitionPerformanceAndRetentionBonuses				$ 100,000
Subsequent Event [Domain] | Anthony Belisle v. Markel CATCo Investment Management Ltd and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims					$ 66,000
Subsequent Event [Domain] | Alissa Fredricks v. Markel CATCo Investment Management Ltd. and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims					$ 7,500