Shareholders of Chicago Mercantile Exchange Holdings Inc. and CBOT Holdings Inc. on Monday voted convincingly to merge the two giant futures exchanges under one roof.

Separate votes at each exchange returned an overwhelming majority in favor of CME's purchase of CBOT, a package worth about $11.9 billion, exchange officials said citing preliminary results.

A vote that once looked like it might be a toss-up, given a higher competing bid for CBOT from IntercontinentalExchange Inc., turned into a landslide.

Today has been a long time in coming, said Terry Duffy, executive chairman of CME.

Analysts had predicted an easy victory for CME after the largest U.S. futures exchange on Friday improved its bid for a third time on Friday to quell dissent among CBOT shareholders.

Caledonia Investments Pty. Ltd., the largest owner of CBOT shares, dropped its opposition to the deal on Friday after the terms were improved.

At that point many CBOT member/shareholders skeptical of the CME deal changed their votes.

CBOT shareholders will receive 0.375 shares of the company's common stock, up from 0.35 in the earlier agreement, for each CBOT share, giving them a 36 percent stake in the combined company, to be known as CME Group.

CME shares closed at $570.58, down $4.22, or 0.7 percent, on the New York Stock Exchange. CBOT closed down $1.18, or 0.5 percent, at $222.82. ICE finished up 69 cents, or 0.4 percent, at $156.78.

(Additional reporting by Christine Stebbins in Chicago, Anupreeta Das and Mark McSherry in New York and Jessica Hall in Philadelphia)