Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2025 MWh MMBTU gal	Sep. 30, 2024 MWh MMBTU gal
Fuel oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	24	23
Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	268	258
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	4	4
Union Electric Company | Fuel oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	24	23
Union Electric Company | Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	47	45
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	0	0
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	221	213
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	4	4