Ameren Corp. is a public utility holding company, which engages in the provision of electric and natural gas services. The company is headquartered in St. Louis, Missouri and currently employs 9,372 full-time employees. The firm operates through four segments: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. The Ameren Missouri segment includes all of the operations of Ameren Missouri. Ameren Missouri operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. The Ameren Illinois Electric Distribution segment consists of the electric distribution business of Ameren Illinois. The Ameren Illinois Natural Gas segment consists of the natural gas business of Ameren Illinois. The Ameren Transmission segment primarily consists of the aggregated electric transmission businesses of Ameren Illinois and Ameren Transmission Company of Illinois (ATXI). The firm owns an integrated transmission system that is composed of the transmission assets of Ameren Missouri, Ameren Illinois and ATXI.