Warren Buffett's Berkshire Hathaway will sell its 16.3 percent stake in White Mountains Insurance Group in a transaction valued at $836 million, seven years after investing in the Bermuda-based insurer, White Mountains said Monday. Berkshire will exchange its 1.724 million shares for about $751 million of cash plus a unit that holds two businesses, Commercial Casualty Insurance and International American Group, with $435 million of assets and $58 million of adjusted shareholder equity, White Mountains said. Thetransaction is valued at $485 per share, it said.