Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	12 Months Ended
	Dec. 31, 2021 MMBTU MWh lb gal	Dec. 31, 2020 MMBTU MWh lb gal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	30	43
Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	179	147
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	12	13
Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	586	365
Union Electric Company | Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	30	43
Union Electric Company | Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	35	33
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	6	6
Union Electric Company | Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	586	365
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	144	114
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	6	7
Ameren Illinois Company | Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	0	0