Related Party Transactions	12 Months Ended
Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	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 investment management operations are included below. Nephila In November 2018, the Company expanded its investment management 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 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 these unconsolidated affiliates 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. Total revenues attributed to the management contracts with the Nephila Funds and the Nephila Reinsurers from the acquisition date to December 31, 2018 were $24.4 million. Within the Company’s program services business, the Company has a program with Nephila and Lloyd’s Syndicate 2357 (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 year ended December 31, 2018, gross premiums written on the Company’s program with Nephila were $322.1 million, all of which was ceded to Syndicate 2357. As of December 31, 2018, reinsurance recoverables on the consolidated balance sheet included $179.8 million due from Syndicate 2357. Under this program, the Company also bears underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is highly 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 highly unlikely, those losses, if incurred, could be material to the Company’s consolidated results of operations and financial condition. Markel CATCo Within the Company's reinsurance operations, the Company enters into reinsurance contracts that are ceded to Markel CATCo Re, an unconsolidated subsidiary. Under this program, the Company retains underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is highly 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 years ended December 31, 2018, 2017 and 2016, gross premiums written on behalf of Markel CATCo Re were $10.9 million, $9.7 million and $6.7 million, respectively. In early 2019, the Company committed to enter into various reinsurance contracts with third parties on behalf of Markel CATCo Re. These reinsurance contracts will primarily cover losses for events that may occur during 2019, however, in some instances, coverage will also provide for adverse development on 2018 and prior accident years’ loss events. Incurred losses on these contracts will be fully ceded to Markel CATCo Re. The loss exposures on these contracts will be fully collateralized by Markel CATCo Re up to an amount that the Company believes is highly unlikely to be exceeded. The Company will have credit risk from Markel CATCo Re for any uncollateralized amounts. Markel CATCo Re's ability to pay losses in excess of the collateralized amounts will depend on the availability of funds that are not otherwise needed to pay losses on other contracts. The Company's maximum exposure to loss on these contracts, representing the net uncollateralized risks, is not expected to exceed $250 million in 2019. See note 16 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 investment management operations are included below.

Nephila

In November 2018, the Company expanded its investment management 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 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 these unconsolidated affiliates 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. Total revenues attributed to the management contracts with the Nephila Funds and the Nephila Reinsurers from the acquisition date to December 31, 2018 were $24.4 million.

Within the Company’s program services business, the Company has a program with Nephila and Lloyd’s Syndicate 2357 (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 year ended December 31, 2018, gross premiums written on the Company’s program with Nephila were $322.1 million, all of which was ceded to Syndicate 2357. As of December 31, 2018, reinsurance recoverables on the consolidated balance sheet included $179.8 million due from Syndicate 2357.

Under this program, the Company also bears underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is highly 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 highly unlikely, those losses, if incurred, could be material to the Company’s consolidated results of operations and financial condition.

Markel CATCo

Within the Company's reinsurance operations, the Company enters into reinsurance contracts that are ceded to Markel CATCo Re, an unconsolidated subsidiary. Under this program, the Company retains underwriting risk for annual aggregate agreement year losses in excess of a limit the Company believes is highly 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 years ended December 31, 2018, 2017 and 2016, gross premiums written on behalf of Markel CATCo Re were $10.9 million, $9.7 million and $6.7 million, respectively.

In early 2019, the Company committed to enter into various reinsurance contracts with third parties on behalf of Markel CATCo Re. These reinsurance contracts will primarily cover losses for events that may occur during 2019, however, in some instances, coverage will also provide for adverse development on 2018 and prior accident years’ loss events. Incurred losses on these contracts will be fully ceded to Markel CATCo Re. The loss exposures on these contracts will be fully collateralized by Markel CATCo Re up to an amount that the Company believes is highly unlikely to be exceeded. The Company will have credit risk from Markel CATCo Re for any uncollateralized amounts. Markel CATCo Re's ability to pay losses in excess of the collateralized amounts will depend on the availability of funds that are not otherwise needed to pay losses on other contracts. The Company's maximum exposure to loss on these contracts, representing the net uncollateralized risks, is not expected to exceed $250 million in 2019.

See note 16 for details of the Company's other transactions with Markel CATCo Re and the Markel CATCo Funds.