LONDON & ARLINGTON, Va.--(BUSINESS WIRE)--Willis Group Holdings (NYSE:WSH) and Towers Watson (NASDAQ:TW) today announced the signing of a definitive merger agreement under which the companies will combine in an all-stock merger of equals transaction. Based on the closing prices of Willis and Towers Watson common stock on June 29, 2015, the implied equity value of the transaction is approximately $18 billion. The transaction has been unanimously approved by the Board of Directors of each company