Bermuda-based White Mountains Insurance Group Ltd announced yesterday that it intends to commence a “modified Dutch auction” self-tender offer to buy up to 300,000 of its common shares.Under the deal the shares would be purchased at a price of not greater than $435 or less than $385 per share net to the seller in cash.The offer is expected to commence on August 18, 2011 and expire at 12am, New York City time, on September 15, 2011, unless extended. White Mountains’ common shares closed on the New York Stock Exchange at $403 per share on Tuesday.A “modified Dutch auction” self-tender offer allows shareholders to indicate how many shares and at what price within the company’s specified range they wish to tender their shares. Based on the number of shares tendered and the prices specified by the tendering shareholders, the company will determine the lowest price per share within the range that will enable it to buy 300,000 shares, or such lesser number of shares that are properly tendered and not properly withdrawn.