Related Party Transactions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Apr. 30, 2011 Ameren Illinois Company Ameren Transmission Company of Illinois	Jan. 31, 2011 Ameren Illinois Company Ameren Transmission Company of Illinois	Apr. 30, 2012 Period Five Union Electric Company	May 31, 2011 Period Five Ameren Illinois Company Union Electric Company MWh	Apr. 30, 2012 Period Six Union Electric Company	May 31, 2011 Period Three Ameren Illinois Company Union Electric Company MWh	Apr. 30, 2010 Period Four Union Electric Company	May 31, 2011 Period Four Ameren Illinois Company Union Electric Company MWh
Related Party Transaction [Line Items]
Energy Supply Agreements Amount			$ 1		$ 3		$ 1
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power				40,800		16,800		40,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate				28		37		29
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress, Proceeds from Advances to Related Party		52
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress, Proceeds from Advances to Related Party, Accrued Interest		3
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress, Transfers from Related Party	$ 20