American Express estimates $1 billion gain from Costco U.S. co-branded card portfolio sale

American Express Co. AXP, +0.56% said in a Monday release it would see a $1 billion gain from the sale of its Costco Wholesale Corp. COST, +1.51% U.S. co-branded card portfolio to Citibank N.A. C, +2.30% The sale is expected to close June 2016, and all eligible Costco American Express Card accounts will be transferred to Citi. Because the close is several months away and card member borrowing and payment trends are hard to determine, the gain could change, American Express said. American Express shares are unchanged in Monday premarket trading, but down 32.1% for the past year. The S&P 500 is down 7.4% for the past 12 months.