The United States government will sell most of its share in AIG after bailing the company out four years ago, the Treasury Department announced Sunday. The $18 billion stock offering will decrease the government’s stake in the company to a 20 percent holding, down from the 53 percent majority share it held in the insurer. AIG plans to purchase back about $5 billion of the stock, the company said. The company will be classified as a savings and loan holding company after the buyback. The Treasury, which purchased the shares for $28.72, stands to make a profit on the deal, with AIG closing at $33.99 on Friday.