Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 11	$ 22
Gross derivative instruments not offset in the balance sheet	7	7
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	4	15
Gross derivative liability amount recognized on the balance sheet	145	168
Gross derivative instruments not offset in the balance sheet	7	7
Gross cash collateral posted not offset in the balance sheet	1	6
Net derivative liability	137	155
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	7	19
Gross derivative instruments not offset in the balance sheet	5	6
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	2	13
Gross derivative liability amount recognized on the balance sheet	20	25
Gross derivative instruments not offset in the balance sheet	5	6
Gross cash collateral posted not offset in the balance sheet	0	0
Net derivative liability	15	19
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	4	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	2	2
Gross derivative liability amount recognized on the balance sheet	125	143
Gross derivative instruments not offset in the balance sheet	2	1
Gross cash collateral posted not offset in the balance sheet	1	6
Net derivative liability	$ 122	$ 136