Related Party Transactions Narrative (Details) - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	9 Months Ended
	Sep. 30, 2015 MWh $ / MWh
April 2015 Procurement [Member]
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 | $ / MWh	36
September 2015 Procurement [Member]
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power	339,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	38