Related Party Transactions Related Party Transactions	3 Months Ended
Mar. 31, 2019
Related Party Transaction [Line Items]
Related Party Transactions Disclosure [Text Block]	Related Party Transactions The Company engages in certain related party transactions in the normal course of business at arm's length. Details of the Company's transactions with related parties in its insurance-linked securities operations are included below. Nephila In November 2018, the Company expanded its insurance-linked securities operations through the acquisition of Nephila, which serves as the investment manager to several Bermuda, Ireland and U.S. based private funds (the Nephila Funds). To provide access for the Nephila Funds to the insurance, reinsurance and weather markets, Nephila also provides managing general agent services and acts as an insurance manager to certain Bermuda Class 3 and 3A reinsurance companies and as both a service company coverholder and agent with binding authority for Lloyd’s Syndicate 2357 (Syndicate 2357) (collectively, the Nephila Reinsurers). Nephila receives management fees for its investment and insurance management services from the Nephila Funds based on the net asset value of the accounts managed, and, for certain funds, incentive fees based on the annual performance of the funds it manages. Nephila also receives commissions from the Nephila Reinsurers, which are based on the direct written premium of the insurance contracts placed. Total revenues attributed to services provided to the Nephila Funds and the Nephila Reinsurers for the three months ended March 31, 2019 were $43.2 million. Within the Company’s program services business, the Company has a program with Nephila and Syndicate 2357, one of its unconsolidated affiliates, through which the Company writes insurance policies that are ceded to Syndicate 2357. Through this arrangement, Nephila has the exclusive right, through 2019, to utilize certain of the Company’s licensed insurance companies to write U.S. catastrophe exposed property risk that will then be ceded to Syndicate 2357. For the three months ended March 31, 2019, gross premiums written through the Company’s program with Nephila were $93.0 million, all of which was ceded to Syndicate 2357. As of March 31, 2019 and December 31, 2018, reinsurance recoverables on the consolidated balance sheet included $189.0 million and $179.8 million due from Syndicate 2357, respectively. Under this program, the Company bears underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is unlikely to be exceeded. To the extent losses under this program exceed the prescribed limit, the Company is obligated to pay such losses to the cedents without recourse to Syndicate 2357. While the Company believes losses under this program are unlikely, those losses, if incurred, could be material to the Company’s consolidated results of operations and financial condition. The Company has also entered into both assumed and ceded reinsurance transactions with the Nephila Reinsurers in the normal course of business, which are not material to the Company's consolidated financial statements. Markel CATCo During the first quarter of 2019, the Company entered into various reinsurance contracts with third parties on behalf of Markel CATCo Re, an unconsolidated subsidiary, in exchange for ceding fees. These reinsurance contracts primarily cover losses for events that may occur during 2019, however, in some instances, coverage is also provided for adverse development on 2018 and prior accident years’ loss events. Incurred losses on these contracts are fully ceded to Markel CATCo Re. The loss exposures on these contracts are fully collateralized by Markel CATCo Re up to an amount that the Company believes is unlikely to be exceeded. The Company has credit risk from Markel CATCo Re for any uncollateralized amounts. Markel CATCo Re's ability to pay losses in excess of the collateralized amounts depends on the availability of funds that are not otherwise needed to pay losses on other contracts. As of March 31, 2019, the Company's maximum exposure to loss on these contracts, representing the net uncollateralized risks, was $191.6 million. Total ceding fees attributed to these contracts are $8.8 million, of which $2.2 million was earned and included in services and other revenues in our consolidated statements of income and other comprehensive income for the three months ended March 31, 2019. Results attributed to these contracts are not included in a reportable segment. Within the Company's reinsurance operations, the Company also enters into reinsurance contracts that are ceded to Markel CATCo Re. Under this program, the Company retains underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is unlikely to be exceeded. To the extent losses under this program exceed the prescribed limit, the Company is obligated to pay such losses to the cedents without recourse to Markel CATCo Re. For the three months ended March 31, 2019 and 2018, gross premiums written and ceded to Markel CATCo Re were $4.0 million and $5.5 million, respectively. See note 13 for details of the Company's other transactions with Markel CATCo Re and the Markel CATCo Funds.

Related Party Transactions

The Company engages in certain related party transactions in the normal course of business at arm's length. Details of the Company's transactions with related parties in its insurance-linked securities operations are included below.

Nephila

In November 2018, the Company expanded its insurance-linked securities operations through the acquisition of Nephila, which serves as the investment manager to several Bermuda, Ireland and U.S. based private funds (the Nephila Funds). To provide access for the Nephila Funds to the insurance, reinsurance and weather markets, Nephila also provides managing general agent services and acts as an insurance manager to certain Bermuda Class 3 and 3A reinsurance companies and as both a service company coverholder and agent with binding authority for Lloyd’s Syndicate 2357 (Syndicate 2357) (collectively, the Nephila Reinsurers). Nephila receives management fees for its investment and insurance management services from the Nephila Funds based on the net asset value of the accounts managed, and, for certain funds, incentive fees based on the annual performance of the funds it manages. Nephila also receives commissions from the Nephila Reinsurers, which are based on the direct written premium of the insurance contracts placed. Total revenues attributed to services provided to the Nephila Funds and the Nephila Reinsurers for the three months ended March 31, 2019 were $43.2 million.

Within the Company’s program services business, the Company has a program with Nephila and Syndicate 2357, one of its unconsolidated affiliates, through which the Company writes insurance policies that are ceded to Syndicate 2357. Through this arrangement, Nephila has the exclusive right, through 2019, to utilize certain of the Company’s licensed insurance companies to write U.S. catastrophe exposed property risk that will then be ceded to Syndicate 2357. For the three months ended March 31, 2019, gross premiums written through the Company’s program with Nephila were $93.0 million, all of which was ceded to Syndicate 2357. As of March 31, 2019 and December 31, 2018, reinsurance recoverables on the consolidated balance sheet included $189.0 million and $179.8 million due from Syndicate 2357, respectively.

Under this program, the Company bears underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is unlikely to be exceeded. To the extent losses under this program exceed the prescribed limit, the Company is obligated to pay such losses to the cedents without recourse to Syndicate 2357. While the Company believes losses under this program are unlikely, those losses, if incurred, could be material to the Company’s consolidated results of operations and financial condition.

The Company has also entered into both assumed and ceded reinsurance transactions with the Nephila Reinsurers in the normal course of business, which are not material to the Company's consolidated financial statements.

Markel CATCo

During the first quarter of 2019, the Company entered into various reinsurance contracts with third parties on behalf of Markel CATCo Re, an unconsolidated subsidiary, in exchange for ceding fees. These reinsurance contracts primarily cover losses for events that may occur during 2019, however, in some instances, coverage is also provided for adverse development on 2018 and prior accident years’ loss events. Incurred losses on these contracts are fully ceded to Markel CATCo Re. The loss exposures on these contracts are fully collateralized by Markel CATCo Re up to an amount that the Company believes is unlikely to be exceeded. The Company has credit risk from Markel CATCo Re for any uncollateralized amounts. Markel CATCo Re's ability to pay losses in excess of the collateralized amounts depends on the availability of funds that are not otherwise needed to pay losses on other contracts. As of March 31, 2019, the Company's maximum exposure to loss on these contracts, representing the net uncollateralized risks, was $191.6 million. Total ceding fees attributed to these contracts are $8.8 million, of which $2.2 million was earned and included in services and other revenues in our consolidated statements of income and other comprehensive income for the three months ended March 31, 2019. Results attributed to these contracts are not included in a reportable segment.

Within the Company's reinsurance operations, the Company also enters into reinsurance contracts that are ceded to Markel CATCo Re. Under this program, the Company retains underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is unlikely to be exceeded. To the extent losses under this program exceed the prescribed limit, the Company is obligated to pay such losses to the cedents without recourse to Markel CATCo Re. For the three months ended March 31, 2019 and 2018, gross premiums written and ceded to Markel CATCo Re were $4.0 million and $5.5 million, respectively.

See note 13 for details of the Company's other transactions with Markel CATCo Re and the Markel CATCo Funds.