Exelon Corporation is an American energy generator and distributor headquartered in the Chase Tower in the Chicago Loop area of Chicago, Illinois, United States. Exelon is the largest operator of nuclear power plants in the United States. It was created in October 2000 by the merger of PECO Energy Company and Unicom, of Philadelphia and Chicago respectively. Unicom owned Commonwealth Edison. Exelon operates regulated utilities in Illinois, Pennsylvania, and Maryland. In October 2009, Exelon had full or majority ownership of 17 nuclear reactors in 10 nuclear power plants. Exelon has operations and business activities in 47 states, the District of Columbia and Canada and is the largest competitive U.S. power generator with approximately 35,000 megawatts of owned capacity. Exelon merged with Constellation Energy Group in March 2012.