Related Party Transactions (Narrative) (Details) - Ameren Illinois Company $ in Millions	1 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Apr. 30, 2020 USD ($)	Sep. 30, 2020 MWh $ / MWh
April 2020 Procurement | Ameren Illinois Capacity Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Energy Supply Agreements Amount		$ 2
September 2020 Procurement | Ameren Illinois Capacity Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	$ 1
September 2020 Procurement | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Related Party Long Term Contract For Purchase of Electric Power | MWh			204,800
Related Party Long Term Contract For Purchase of Electric Power Rate | $ / MWh			31