Related Party Transactions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended			12 Months Ended
	Apr. 30, 2012	May 31, 2011 MWh	Apr. 30, 2010	Dec. 31, 2014 MWh
Period Five | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	$ 1
Period Five | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power		40,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate		28
Period Six | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	3
Period Three | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power		16,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate		37
Period Four | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount			$ 1
Period Four | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power		40,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate		29
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power				168,400
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate				51