Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 39	$ 89
Gross derivative instruments not offset in the balance sheet	20	29
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	19	60
Gross derivative liability amount recognized on the balance sheet	130	136
Gross derivative instruments not offset in the balance sheet	20	29
Gross cash collateral posted not offset in the balance sheet	16	56
Net derivative liability	94	51
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	25	49
Gross derivative instruments not offset in the balance sheet	9	9
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	16	40
Gross derivative liability amount recognized on the balance sheet	32	68
Gross derivative instruments not offset in the balance sheet	9	9
Gross cash collateral posted not offset in the balance sheet	16	56
Net derivative liability	7	3
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	14	40
Gross derivative instruments not offset in the balance sheet	11	20
Gross cash collateral received not offset in the balance sheet	0	0
Net derivative asset	3	20
Gross derivative liability amount recognized on the balance sheet	98	68
Gross derivative instruments not offset in the balance sheet	11	20
Gross cash collateral posted not offset in the balance sheet	0	0
Net derivative liability	$ 87	$ 48