WARRENVILLE, ILL. — Fairbridge Properties has acquired Cornerstone I at Cantera, a 152,520-square-foot office building in Warrenville, about 30 miles west of Chicago, for $20.7 million. The Class A building is part of the 650-acre mixed-use development known as Cantera. The seller, Northwestern Mutual, invested $540,000 in fitness center and conference room renovations. The building is currently 79.6 percent leased. Jim Postweiler, Peter Harwood and Derek Fohl of JLL represented Northwestern Mutual in the sale.