(Bloomberg) -- Jana Partners has amassed a stake in insurer Markel Group Inc. and is pushing the specialty insurance company to explore a separation or sale of its private investments business, according to Scott Ostfeld, managing partner at the activist investment firm.

Markel is “significantly undervalued” and would benefit from improvement in execution of its core insurance business, Ostfeld said on Tuesday. Jana believes that the entire company is an “attractive” takeover target for larger insurers. 

The size of Jana’s stake in Markel wasn’t disclosed. A representative for Markel didn’t immediately respond to a request for comment. 

Shares of Glen Allen, Virginia-based Markel have jumped 24% this year, giving the company a market value of almost $23 billion. The stock performance trails the Dow Jones US Property and Casualty Insurance Index, which is up 36% this year. 

Markel’s core business is specialty insurance, where underwrites policies for individuals and businesses in the US, UK, European Union, Asia and Australia, according to its annual report. It also has an investment arm and private equity-like division called Markel Venture, whose holdings include luxury handbag brand Brahmin and crawler crane firm Buckner Heavylift Cranes, according to its website.

This year, New York-based Jana has taken stakes and pushed for changes at companies including food processing group Lamb Weston Holdings Inc. and software provider Rapid7 Inc. 

Jana also urged Frontier Communications Parent Inc. to explore a sale before Verizon Communications Inc. acquired the company in a $19.6 billion deal that was approved by shareholders last month.

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