Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 3,655
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	2,145
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 1,444
April 2017 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	4,249,800
Long-term Purchase Commitment, Amount	$ 128