Exelon Corporation is an American Fortune 100 energy company headquartered in Chicago, Illinois. It generates revenues of approximately $33.5 billion and employs approximately 34,000 people. Exelon is the largest electric parent company in the United States by revenue, the largest regulated utility in the United States with approximately 10 million customers, and 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 of Philadelphia and Unicom Corp of Chicago, which 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 23 nuclear reactors in 14 nuclear power plants. Exelon has operations and business activities in 48 states, the District of Columbia and Canada, and is the largest competitive U.S. power generator with approximately 35,500 megawatts of owned capacity. Exelon merged with Constellation Energy Group in March 2012 and acquired Pepco Holdings in March 2016. Exelon consists of eight main operating subsidiaries with Exelon Generation, a deregulated energy generator, Constellation Energy Group, a deregulated competitive energy supplier, and six regulated utilities, Commonwealth Edison (Illinois), PECO Energy Company (Pennsylvania), Baltimore Gas and Electric (Maryland), Delmarva Power & Light (Delaware and Maryland), Atlantic City Electric (New Jersey), and Potomac Electric Power Company (Washington, DC and Maryland).