Recent Developments Related to COVID-19 Recent Developments Related to COVID-19	6 Months Ended
	Jun. 30, 2020
Unusual Risk or Uncertainty [Line Items]
Recent Developments Related to COVID-19 [Text Block]	Recent Developments Related to COVID-19 The COVID-19 pandemic has caused unprecedented social and economic disruption, increased volatility of capital markets and intervention by various governments and central banks around the world, the effects of which have impacted almost all of the Company's operations during the first half of 2020. The Company cannot reasonably estimate the extent or duration of the impacts of the pandemic; however, further potential impacts of the pandemic on the Company's results of operations, financial condition and cash flows, including those described below, could be material. The significant volatility in the equity markets arising from economic uncertainty resulted in a decline in the fair value of the Company's equity portfolio of $777.1 million for the six months ended June 30, 2020 and further declines are possible. As described in note 9, the Company's underwriting results for the six months ended June 30, 2020 included $325.0 million of net losses and loss adjustment expenses directly attributed to COVID-19 and assumptions used to develop this estimate are inherently uncertain and subject to a wide range of variability. As efforts to respond to the pandemic continue to evolve, the Company expects that losses indirectly related to the COVID-19 pandemic and associated with a broader range of coverages are likely to emerge within the Company's professional liability, trade credit and workers' compensation product lines, among others, including the Company's reinsurance product lines, though no material losses have been reported at this time. Business closures, reduced recreational activity and lower gross receipts, revenues and payrolls of insureds, among other things, also may impact the Company's premium volume and the economic impacts of the pandemic on the Company's insureds also may subject it to increased credit risk. A significant decline in economic activity also could impact premium volume within the Company's program services operations, which may result in a reduction in fee income. Within the Company's Markel Ventures operations, many of the Company's businesses have experienced decreased demand for their products and services as a result of the pandemic. As the social and economic disruption caused by the pandemic is ongoing, the Company expects that revenues from its Markel Ventures operations will continue to be impacted, and these impacts may continue to be material. In certain cases, revenue declines also could result in ongoing cash and working capital constraints and could impact the companies’ liquidity and their ability to comply with debt covenants. Within the Company's insurance-linked securities operations, investment losses to date within the investment funds managed by the Company have not been significant; however, uncertainty around potential COVID-19 loss exposures has reduced, and may further reduce, the net asset value on which the Company's management fees are based. Volatility in the capital markets and investor uncertainty regarding insurance industry exposure to COVID-19 also has impacted, and may continue to impact, the Company's ability to raise additional third party capital for the funds it manages. The Company also has experienced, and may continue to experience, higher than anticipated investor redemptions from the funds. Loss of revenues in the Company's underwriting, Markel Ventures, insurance-linked securities or other operations, the extent of which the Company is currently unable to estimate, also could impact the carrying value of the Company's goodwill and intangible assets and, with respect to its Markel Ventures operations, inventory and other long-lived assets, which may become impaired. The Company's consolidated balance sheet as of June 30, 2020 included goodwill and intangible assets of $4.5 billion. Through June 30, 2020, the Company considered whether a quantitative assessment of goodwill and intangible assets for impairment was required as a result of the significant economic disruption caused by the COVID-19 pandemic. After considering qualitative factors regarding the actual and expected impacts of the pandemic on the Company's operations, as well as the amount by which the fair value of the Company's reporting units exceeded their respective carrying values at the date of the last quantitative assessment, the Company determined these conditions did not indicate that it is more likely than not that the carrying value of the Company's reporting units exceeded their fair value as of June 30, 2020 based on information available at this time. Similar factors were considered to determine if these circumstances were an indicator requiring an assessment of the recoverability of the Company's intangible assets, and the Company concluded they were not based on information available at this time. However, delayed recovery or further deterioration in market conditions related to the general economy and the specific industries in which the Company operates, a sustained trend of weaker than anticipated financial performance within a reporting unit, or an increase in the market-based weighted average cost of capital, among other factors, could significantly impact the impairment analysis and may result in future goodwill or intangible asset impairment charges that, if incurred, could have a material adverse effect on the Company's financial condition and results of operations.