NEW YORK (Bloomberg) ? OneBeacon Insurance Group Ltd., the biggest unit of White Mountains Insurance Group Ltd., saw its shares rise four percent on their first day of trading on the New York Stock Exchange.

OneBeacon raised $600 million in an initial public stock sale on Wednesday night after increasing its offering by 20 percent.

OneBeacon sold 24 million shares at $25 each, the company said in a statement. The Bermuda-based insurer said it would sell 20 million shares at $24 to $26 apiece in a filing with regulators in October.

although the shares failed to reach the top of their range in the IPO they took off yesterday, reaching $26.50 in trading at around 3 p.m. before slipping to $26.01 at the close.

About 12.75 million shares traded.

The IPO sale valued OneBeacon at about $2.5 billion. White Mountains will keep the proceeds and hold roughly 76 percent of OneBeacon?s stock.

OneBeacon sold shares in a year when US insurers are expected to post record annual profits. The industry is poised to earn $55 billion to $60 billion of net income in 2006 after insurers passed the peak of the Atlantic Ocean hurricane season unscathed, according to an October estimate by Bob Hartwig, chief economist at the Insurance Information Institute in New York.

In its regulatory filings, OneBeacon said its board plans to approve a dividend of 21 cents a share per quarter, beginning in the first quarter of 2007.

Eight members of the board will be current or former employees, directors or officers of White Mountains, the company said.

White Mountains, chaired by Jack Byrne, former chief executive officer of Geico Corp., purchased OneBeacon for $2.17 billion in June, 2001. The insurer purchased the company from the UK?s largest insurer, Aviva Plc.

Billionaire Warren Buffett invested $300 million in White Mountains at the time as part of the deal to finance the purchase. Buffett?s company, Berkshire Hathaway Inc., is now White Mountains? second-biggest shareholder, with 16 percent of the stock, according to Bloomberg data last updated in March.

OneBeacon?s shares will trade on the New York Stock Exchange under the ticker OB. Underwriters were led by Lehman Brothers Inc.