Variable Interest Entities	6 Months Ended
Jun. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Variable Interest Entities	Variable Interest Entities Markel CATCo Investment Management Ltd. (MCIM), a wholly-owned consolidated subsidiary of the Company, is an insurance-linked securities investment fund manager and insurance manager headquartered in Bermuda. Results attributable to MCIM are included with the Company's non-insurance operations, which are not included in a reportable segment. MCIM manages a mutual fund company and reinsurance company, both of which were organized under Bermuda law. The mutual fund company issues multiple classes of nonvoting, redeemable preference shares to investors through its funds (the Funds) and the Funds are primarily invested in nonvoting shares of the reinsurance company. The underwriting results of the reinsurance company are attributed to the Funds through the issuance of nonvoting preference shares. The Funds and the reinsurance company are considered VIEs, as their preference shareholders have no voting rights. MCIM has the power to direct the activities that most significantly impact the economic performance of these entities, but does not have a variable interest in any of the entities. Except as described below, the Company is not the primary beneficiary of the Funds or the reinsurance company, as the Company's involvement is generally limited to that of an investment or insurance manager, receiving fees that are at market and commensurate with the level of effort required. Investment management fees earned by the Company from unconsolidated Funds were $9.3 million and $7.4 million for the quarters ended June 30, 2017 and 2016, respectively, and $18.6 million and $14.5 million for the six months ended June 30, 2017 and 2016, respectively. The Company is the sole investor in one of the Funds, the Markel Diversified Fund, and consolidates that fund as its primary beneficiary. As of June 30, 2017, total assets of the Markel Diversified Fund were $186.0 million and total liabilities were $63.8 million. As of December 31, 2016, total assets of the Markel Diversified Fund were $166.8 million and total liabilities were $64.6 million. The assets of the Markel Diversified Fund are available for use only by the Markel Diversified Fund, and are not available for use by the Company. Total assets of the Markel Diversified Fund include an investment in one of the unconsolidated Funds totaling $183.3 million as of June 30, 2017 and $165.1 million as of December 31, 2016, which represents 5% of the outstanding preference shares of that fund as of June 30, 2017 and 6% as of December 31, 2016. This investment is included in equity securities (available-for-sale) on the Company's consolidated balance sheets. Total liabilities of the Markel Diversified Fund for both periods includes a $62.5 million note payable, delivered as part of the consideration provided for its investment. This note payable is included in senior long-term debt and other debt on the Company's consolidated balance sheets. Other than the note payable, any liabilities held by the Markel Diversified Fund have no recourse to the Company's general credit. The Company's exposure to risk from the unconsolidated Funds and reinsurance company is generally limited to its investment and any earned but uncollected fees. The Company has not issued any investment performance guarantees to these VIEs or their investors. As of June 30, 2017, total investment and insurance assets under management of MCIM for unconsolidated VIEs were $4.4 billion.

Variable Interest Entities

Markel CATCo Investment Management Ltd. (MCIM), a wholly-owned consolidated subsidiary of the Company, is an insurance-linked securities investment fund manager and insurance manager headquartered in Bermuda. Results attributable to MCIM are included with the Company's non-insurance operations, which are not included in a reportable segment.

MCIM manages a mutual fund company and reinsurance company, both of which were organized under Bermuda law. The mutual fund company issues multiple classes of nonvoting, redeemable preference shares to investors through its funds (the Funds) and the Funds are primarily invested in nonvoting shares of the reinsurance company. The underwriting results of the reinsurance company are attributed to the Funds through the issuance of nonvoting preference shares.

The Funds and the reinsurance company are considered VIEs, as their preference shareholders have no voting rights. MCIM has the power to direct the activities that most significantly impact the economic performance of these entities, but does not have a variable interest in any of the entities. Except as described below, the Company is not the primary beneficiary of the Funds or the reinsurance company, as the Company's involvement is generally limited to that of an investment or insurance manager, receiving fees that are at market and commensurate with the level of effort required. Investment management fees earned by the Company from unconsolidated Funds were $9.3 million and $7.4 million for the quarters ended June 30, 2017 and 2016, respectively, and $18.6 million and $14.5 million for the six months ended June 30, 2017 and 2016, respectively. The Company is the sole investor in one of the Funds, the Markel Diversified Fund, and consolidates that fund as its primary beneficiary.

As of June 30, 2017, total assets of the Markel Diversified Fund were $186.0 million and total liabilities were $63.8 million. As of December 31, 2016, total assets of the Markel Diversified Fund were $166.8 million and total liabilities were $64.6 million. The assets of the Markel Diversified Fund are available for use only by the Markel Diversified Fund, and are not available for use by the Company. Total assets of the Markel Diversified Fund include an investment in one of the unconsolidated Funds totaling $183.3 million as of June 30, 2017 and $165.1 million as of December 31, 2016, which represents 5% of the outstanding preference shares of that fund as of June 30, 2017 and 6% as of December 31, 2016. This investment is included in equity securities (available-for-sale) on the Company's consolidated balance sheets. Total liabilities of the Markel Diversified Fund for both periods includes a $62.5 million note payable, delivered as part of the consideration provided for its investment. This note payable is included in senior long-term debt and other debt on the Company's consolidated balance sheets. Other than the note payable, any liabilities held by the Markel Diversified Fund have no recourse to the Company's general credit.

The Company's exposure to risk from the unconsolidated Funds and reinsurance company is generally limited to its investment and any earned but uncollected fees. The Company has not issued any investment performance guarantees to these VIEs or their investors. As of June 30, 2017, total investment and insurance assets under management of MCIM for unconsolidated VIEs were $4.4 billion.