Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 4,566
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	2,786
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 1,729
April 2016 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	3,609,800
Long-term Purchase Commitment, Amount	$ 105