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, 2020 MMBTU MWh lb gal	Dec. 31, 2019 MWh MMBTU lb gal
Fuel Oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	43	58
Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	147	156
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	13	12
Uranium
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	43	58
Union Electric Company | Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	33	20
Union Electric Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	6	5
Union Electric Company | Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	365	565
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	136
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	7	7
Ameren Illinois Company | Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | lb	0	0