Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	3 Months Ended
	Mar. 31, 2020 MWh MMBTU lb gal	Mar. 31, 2019 MWh MMBTU lb gal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	62	58
Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	167	156
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	12	12
Uranium [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	365	565
Union Electric Company | Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	62	58
Union Electric Company | Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	20	20
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	5	5
Union Electric Company | Uranium [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	365	565
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	147	136
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	7