Shares of Bermuda-based White Mountains Insurance Group Ltd soared 15 percent on the New York Stock Exchange yesterday after the company agreed to sell its Esurance unit to Allstate Corp in a $1 billion deal.Esurance is one of the largest sellers of car insurance via the internet and call centres in the US. Also included in the sale was Answer Financial, an online comparison engine that lets users get quotes and buy insurance from dozens of carriers. The terms of the deal mean the sale will earn White Mountains $700 million in cash, plus the tangible book value of the entities at the closing of the sale, expected to be before the end of this year.White Mountains expects the transaction will increase its adjusted book value by approximately $80 per share.Yesterday, White Mountains shares rose $51.36 to close on $394.99.In a statement released yesterday, White Mountains chairman and CEO Raymond Barrette said: “Over the past ten years, Gary Tolman and his team have done a phenomenal job building a start-up into one of the top online insurers in the United States.“We have happily invested significant capital in what we believed was a unique business. Allstate agrees and can now take it to a new level. This is a win-win transaction. We thank Gary and team for a great ride and we wish them continued success as they transition to Allstate.”Esurance’s gross premiums written and policies in force both rose six percent in the first quarter. The company, which claims to be the third-largest online auto insurer, has been advertising aggressively, often using an animated format designed to appeal to young people.Allstate will hope the acquisition will help it to reverse its trend of shrinking car insurance premiums. Premiums written fell one percent last quarter, and policies in force declined as fewer customers renewed their coverage. Allstate CEO Thomas Wilson said: “Esurance will expand our ability to serve customers that are more self-directed but still prefer a branded product.”Aite group senior property and casualty insurance industry analyst Stephen Applebaum said: “This is a significant acquisition for Allstate, not in terms of any immediate market share growth, but because it will in time enhance Allstate’s ability to support multiple distribution channels and offer an independent auto quote service to better compete with Progressive and Geico.“It should also improve their level of overall innovation through the use of emerging technologies in direct online sales and in claims and allow them to increase market share in auto, property and life insurance with the important, younger ‘millennial’ demographic.“Allstate has been gradually losing auto insurance market share over the past few years and now holds about a 10.5 percent share of the auto insurance market, with about 12 million policyholders and 18 million vehicles. Esurance holds under a 0.2 percent share.”