American Express will sell its Costco U.S. co-branded card portfolio to Citibank. The transaction is expected to close in June 2016. American Express estimates it will book a gain of approximately $1 billion above the value of the outstandings in the Costco portfolio, which were $13.75 billion as of December 31, 2015. When the sale is completed, all eligible Costco/Amex co-branded accounts will be converted to Visa cards issued by Citi. Costco had been in an exclusive credit card issuing and acceptance contract with American Express since 2000. In March 2015, Costco announced an exclusive n...