Bermuda-based White Mountains Insurance Group Ltd has agreed to sell its Esurance and Answer Financial units to US giant Allstate Corp for around $1 billion.Esurance is one of the largest sellers of car insurance via the Internet and call centres in the US, while Answer Financial is a major personal insurance agency.In a statement released this morning, 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.”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.Allstate CEO Thomas Wilson said: “Esurance will expand our ability to serve customers that are more self-directed but still prefer a branded product.”