Ameren Corporation (Ameren) utility holding company. The Company�s principal subsidiaries are Union Electric Company (Ameren Missouri), Ameren Illinois Company (Ameren Illinois) and Ameren Energy Resources Company, LLC (AER). The Company operates in three segments: Ameren Missouri, Ameren Illinois, and Merchant Generation. Ameren Missouri operates a operates a rate-regulated electric generation, transmission and distribution business, and a rate-regulated natural gas transmission and distribution business in Missouri. Ameren Illinois operates a rate-regulated electric and natural gas transmission and distribution business in Illinois. AER consists of non-rate-regulated operations, including Ameren Energy Generating Company (Genco), AmerenEnergy Resources Generating Company (AERG) and Ameren Energy Marketing Company (Marketing Company).