Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 4,832
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	2,913
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 1,888
April 2015 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	5,526,000
Long-term Purchase Commitment, Amount	$ 185
September 2015 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	2,592,000
Long-term Purchase Commitment, Amount	$ 83