Related Party Transactions Narrative (Details) (Ameren Illinois Company, Ameren Illinois Power Supply Agreements with Ameren Missouri)	3 Months Ended
	Mar. 31, 2015 MWh
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power	667,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate	36