Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021 MMBTU MWh gal	Dec. 31, 2020 MWh MMBTU gal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	36	43
Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	148	147
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	14	13
Union Electric Company | Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	36	43
Union Electric Company | Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	34	33
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	6
Ameren Illinois Company | Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	0	0
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	114	114
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	7