Related Party Transactions Narrative (Details) - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	9 Months Ended
	Sep. 30, 2016 MWh $ / MWh
April 2016 Procurement [Member]
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	375,200
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	34.71
September 2016 Procurement [Member]
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	82,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	34.35