Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 2,154
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	1,509
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 627
April 2018 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	3,956,200
Long-term Purchase Commitment, Amount	$ 112
Purchased Power | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 10