Ameren Corporation (Ameren) is a utility holding company. The Company’s principal subsidiaries are Union Electric Company (Ameren Missouri) and Ameren Illinois Company (Ameren Illinois). The Company's segments include Ameren Missouri and Ameren Illinois. Ameren Missouri 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). In December 2013, the Company announced that it has completed the divestiture of its merchant generation business, formerly known as Ameren Energy Resources Company, LLC (AER).