Willis is currently based out of 10 South LaSalle St., One East Wacker Dr. and 222 North Riverside Plaza in Chicago, in addition to its suburban offices in Oak Brook, Lombard and Schaumburg.

"We are very pleased with the financial terms of this new lease," a spokesman for the tower tells GlobeSt.com. "The details that have been disclosed are not a complete picture of the agreement. We view the economic terms to be proprietary information, and do not reveal those for any of our transactions. We are very excited about the new Willis Tower and look forward to welcoming the company here this summer."

The lease, which according to reports gives Willis the space at $14.50 per square foot, is a coup for the building, which recently learned it will soon lose one of its largest tenants. In December, accounting firm Ernst & Young finalized negotiations to take 203,618 square feet on floors 19 through 26 in the John Buck Co.'s new 46-story tower at 155 N. Wacker. The firm is expected to take occupancy in the 1.1-million-square-foot building at its completion in July, though its lease for around 380,000 square feet at the Sears Tower runs through 2012.

A source familiar with the deal says it was unexpected that Willis would choose to locate in the Sears Tower, given the large existing presence of a competitor, insurance company Chubb. Other Chicago executives expressed surprise that the building would be renamed for Willis, as its new office will only make up less than 4% of the 3.8-million-square-foot building.

Built in 1973, the Sears Tower is currently about 82% occupied. Asking lease rates there range from $15 to $20 per square foot, net, depending on location in the building, says Mike Kazmierczak, who represented the building in this deal as SVP with US Equities Asset Management LLC, the building's management and leasing agent.

Earlier this month, IMC Chicago tripled its space with a lease expansion to 52,000 square feet in the Sears Tower. The proprietary trading firm, which will occupy the entire 43rd floor, also extended its lease through 2020 in the deal. As GlobeSt.com reported, this move represents the firm's fourth expansion since starting with 6,500 square feet in the building just over two years ago.

The West Loop property is owned by 233 S. Wacker LLC, which includes American Landmark Properties, the Chetrit Group and the Moinian Group. The West Loop's occupancy rate is below 88% according to Cushman & Wakefield's year-end office market report.