Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details) gal in Millions, MWh in Millions, MMBTU in Millions	Dec. 31, 2018 MWh gal MMBTU	Dec. 31, 2017 MWh gal MMBTU
Fuel Oils
Derivative [Line Items]
Quantity | gal	66	28
Natural gas
Derivative [Line Items]
Quantity | MMBTU	173	163
Power
Derivative [Line Items]
Quantity | MWh	9	12
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	66	28
Union Electric Company | Natural gas
Derivative [Line Items]
Quantity | MMBTU	19	24
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	1	3
Ameren Illinois Company | Natural gas
Derivative [Line Items]
Quantity | MMBTU	154	139
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	8	9