Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) MWh MW
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 4,328
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	2,428
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 1,859
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
September 2016 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	2,229,800
Long-term Purchase Commitment, Amount	$ 71
September 2016 Procurement - Capacity [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment, Amount	$ 96
Amount of Megawatts | MW	1,854