Ameren Illinois Co an Illinois corporation is a subsidiary of Ameren Corporation. It operates a rate-regulated electric and natural gas transmission and distribution business in Illinois. Ameren Illinois has three segments: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. Ameren Illinois Electric Distribution consists of the electric distribution business of Ameren Illinois. Ameren Illinois Natural Gas consists of the natural gas business of Ameren Illinois. Ameren Transmission primarily consists of the aggregated electric transmission businesses of Ameren Illinois and ATXI. The company makes majority of its revenue from Ameren Illinois Electric Distribution segment.