The insurer Willis Group Holdings PLC is buying Towers Watson in an all-stock deal valued at about $18 billion. The combined company will be called Willis Towers Watson and will have about $8.2 billion in revenue and approximately 39,000 employees. Willis shareholders will own approximately 50.1 percent, and Towers Watson shareholders will own about 49.9 percent of the business. The company will be based in Ireland, where Willis has its headquarters. Towers Watson is based in Arlington, Virginia. Willis and Towers expect between $100 million and $125 million in cost savings within three years of closing. Shareholders of financial services company Towers Watson & Co. will receive 2.6490 Willis shares for each Towers share. They will also receive a one-time cash dividend of $4.87 per Towers share. Towers Watson Chairman and CEO John Haley will serve as CEO, while Willis CEO Dominic Casserley will become president and deputy CEO. The new board will have 12 members, with six directors being nominated by each company. The boards of both companies unanimously approved the deal, which is targeted to close by year's end.