Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	Sep. 30, 2018 MWh gal MMBTU	Dec. 31, 2017 MWh gal MMBTU
Fuel Oils
Derivative [Line Items]
Quantity | gal	42	28
Natural Gas
Derivative [Line Items]
Quantity | MMBTU	180	163
Power
Derivative [Line Items]
Quantity | MWh	10	12
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	42	28
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	20	24
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	2	3
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	160	139
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	8	9