Exelon Corporation is an American Fortune 100 energy company headquartered in the Chase Tower in the Chicago Loop area of Chicago, Illinois, United States with revenues of approximately $34.5 billion and employing approximately 34,000 employees. Exelon is the largest electric holding company in the United States by revenue, the largest regulated utility in the United States with approximately 10 million customers, and is also 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, Maryland, Delaware, New Jersey, and Washington, DC. In October 2009, Exelon had full or majority ownership of 17 nuclear reactors in 10 nuclear power plants.