(Bloomberg) ? White Mountains Insurance Group Ltd. said it will raise as much as $500 million by selling shares of unit OneBeacon Insurance Group in an initial public offering.

White Mountains will list the shares as ?OB? on the New York Stock Exchange, the Bermuda-based company said in a statement last week.

OneBeacon, based in Boston, sells home and auto insurance as well as coverage for industries including shipping and farming.

Shares of White Mountains rose $12.88, or 2.7 percent, to $488.88 at 12:52 p.m. in New York Stock Exchange composite trading.

Warren Buffett?s Berkshire Hathaway Inc. is its second- biggest shareholder, holding 16 percent of the company?s stock as of March, according to Bloomberg data. Lehman Brothers Inc. is managing the sale with assistance from Banc of America Securities LLC.

The $500 million maximum includes any shares the banks may purchase.