Acquisitions (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended					3 Months Ended		6 Months Ended		6 Months Ended		12 Months Ended			0 Months Ended
	Jun. 30, 2014 May 1, 2013 Alterra Capital Holdings [Member]	Jun. 30, 2013 May 1, 2013 Alterra Capital Holdings [Member]	Jun. 30, 2013 May 1, 2013 Alterra Capital Holdings [Member] U.S. Insurance Segment [Member]	Jun. 30, 2013 May 1, 2013 Alterra Capital Holdings [Member] International Insurance Segment [Member]	Jun. 30, 2013 May 1, 2013 Alterra Capital Holdings [Member] Reinsurance Segment [Member]	Mar. 31, 2014 January 17, 2014 Abbey Protection plc [Member]	Jan. 17, 2014 January 17, 2014 Abbey Protection plc [Member]	Jun. 30, 2014 January 17, 2014 Abbey Protection plc [Member] Customer Relationships [Member]	Jan. 17, 2014 January 17, 2014 Abbey Protection plc [Member] Customer Relationships [Member]	Jun. 30, 2014 January 17, 2014 Abbey Protection plc [Member] Trade Names [Member]	Jan. 17, 2014 January 17, 2014 Abbey Protection plc [Member] Trade Names [Member]	Dec. 31, 2013 January 17, 2014 Abbey Protection plc [Member] Minimum [Member]	Jan. 17, 2014 January 17, 2014 Abbey Protection plc [Member] International Insurance Segment [Member]	Jan. 17, 2014 January 17, 2014 Abbey Protection plc [Member] Non-Insurance Operations [Member]	Jul. 23, 2014 July 23, 2014 Cottrell Inc [Member] Subsequent Event [Member]	Jul. 23, 2014 July 23, 2014 Cottrell Inc [Member] Subsequent Event [Member]
Business Acquisition [Line Items]
Percentage acquired		100.00%					100.00%									100.00%
Number of Markel Corporation shares issued for each outstanding share of Alterra common stock		0.04315
Cash paid for each outstanding share of Alterra common stock		$ 10.00
Total purchase consideration		$ 3,300.0				$ 190.7
Cash consideration		964.3				190.7									130.0
Stock consideration, shares		4,300,000
Goodwill recognized		295.7	107.8	89.8	98.1		52.8						37.5	15.3
Goodwill, expected tax deductible		0					0
Indefinite lived intangible assets		37.5
Other intangible assets		170.0					130.6		119.9		10.7
Other intangible assets, weighted average amortization period, years	17 years							20 years		14 years
Premiums under management												$ 60