Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	Jun. 30, 2019 MWh gal MMBTU	Dec. 31, 2018 MWh gal MMBTU
Fuel Oils
Derivative [Line Items]
Quantity | gal	67	66
Natural Gas
Derivative [Line Items]
Quantity | MMBTU	163	173
Power
Derivative [Line Items]
Quantity | MWh	12	9
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	67	66
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	17	19
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	4	1
Ameren Illinois Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	0	0
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	146	154
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	8	8