Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	Mar. 31, 2019 MWh gal MMBTU	Dec. 31, 2018 MWh gal MMBTU
Fuel Oils
Derivative [Line Items]
Quantity | gal	65	66
Natural Gas
Derivative [Line Items]
Quantity | MMBTU	174	173
Power
Derivative [Line Items]
Quantity | MWh	10	9
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	65	66
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	18	19
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	2	1
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	156	154
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	8	8