He estimates the full development cost "could go over $1 billion," and says Trinity would likely partner with others on the hotel and residential components.

"The deal is structured to allow us to take our time, and we plan to be very patient in the process," he says. "We'll work with the city and all interested groups, including Thomas Jefferson Hospital, the Convention Center and others." The block currently contains four buildings aggregating 850,000 sf, which Kearns says is 90% leased.

It includes the two-story 210,000-sf Market Street building; the 13-story, 110,000-sf Stephen Girard office building, which was built in the late 1800s and has recognized historical significance; the 110,000-sf Family Court office building, and a garage on Chestnut Street which has some office and retail space. Although there is no timeline, first on Trinity's agenda will be the renovation of the Market Street building, which Kearns says, "needs a lot of work." The building will remain retail.

Proceeds from the lease go to Girard College, also part of the estate. "We couldn't be happier with this transaction, not only for the clear benefits it produces for Girard College, but also for the positive impact it will have for Philadelphia," says John Egan, president of the board of city trusts, in a statement. Paul Levy, executive director of Center City District, also applauded the deal. The CCD recently unveiled what it called a dream plan for revitalization of Girard Square and other parcels along Market Street East.