Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	Dec. 31, 2017 gal MWh MMBTU lb	Dec. 31, 2016 gal MWh MMBTU lb
Fuel Oils
Derivative [Line Items]
Quantity | gal	28	30
Natural Gas
Derivative [Line Items]
Quantity | MMBTU	163	154
Power
Derivative [Line Items]
Quantity | MWh	12	10
Uranium
Derivative [Line Items]
Quantity | lb	370	345
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	28	30
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	24	25
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	3	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb	370	345
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	139	129
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	9	9