Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 35	$ 89
Gross derivative instruments not offset in the balance sheet	11	29
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	24	60
Gross derivative liability amount recognized on the balance sheet	125	136
Gross derivative instruments not offset in the balance sheet	11	29
Gross cash collateral posted not offset in the balance sheet	5	56
Net derivative liability	109	51
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	26	49
Gross derivative instruments not offset in the balance sheet	6	9
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	20	40
Gross derivative liability amount recognized on the balance sheet	20	68
Gross derivative instruments not offset in the balance sheet	6	9
Gross cash collateral posted not offset in the balance sheet	5	56
Net derivative liability	9	3
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	9	40
Gross derivative instruments not offset in the balance sheet	5	20
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	4	20
Gross derivative liability amount recognized on the balance sheet	105	68
Gross derivative instruments not offset in the balance sheet	5	20
Gross cash collateral posted not offset in the balance sheet	0	0
Net derivative liability	$ 100	$ 48