Ameren Illinois Company, doing business as Ameren Illinois, engages in the rate-regulated electric generation, transmission, and distribution business in Illinois. The company also engages in the natural gas distribution businesses. It supplies electric services to 1.2 million customers and natural gas services to 0.8 million customers in a 43,700 square-mile area in central and eastern Illinois. The company was formerly known as Central Illinois Public Service Company and changed its name to Ameren Illinois Company in October 2010. Ameren Illinois Company was founded in 1902 and is based in Collinsville, Illinois. Ameren Illinois Company is a subsidiary of Ameren Corporation.