Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 16	$ 22
Gross derivative instruments not offset in the balance sheet	5	7
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	11	15
Gross derivative liability amount recognized on the balance sheet	156	168
Gross derivative instruments not offset in the balance sheet	5	7
Gross cash collateral posted not offset in the balance sheet	0	6
Net derivative liability	151	155
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	13	19
Gross derivative instruments not offset in the balance sheet	3	6
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	10	13
Gross derivative liability amount recognized on the balance sheet	21	25
Gross derivative instruments not offset in the balance sheet	3	6
Gross cash collateral posted not offset in the balance sheet	0	0
Net derivative liability	18	19
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	3	3
Gross derivative instruments not offset in the balance sheet	2	1
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	1	2
Gross derivative liability amount recognized on the balance sheet	135	143
Gross derivative instruments not offset in the balance sheet	2	1
Gross cash collateral posted not offset in the balance sheet	0	6
Net derivative liability	$ 133	$ 136