Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 180	$ 90
Gross derivative instruments not offset in the balance sheet	25	19
Gross cash collateral received not offset in the balance sheet	14	0
Net derivative asset	141	71
Gross derivative liability amount recognized on the balance sheet	218	202
Gross derivative instruments not offset in the balance sheet	25	19
Gross cash collateral posted not offset in the balance sheet	92	47
Net derivative liability	101	136
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	78	49
Gross derivative instruments not offset in the balance sheet	19	15
Gross cash collateral received not offset in the balance sheet	5	0
Net derivative asset	54	34
Gross derivative liability amount recognized on the balance sheet	136	77
Gross derivative instruments not offset in the balance sheet	19	15
Gross cash collateral posted not offset in the balance sheet	92	47
Net derivative liability	25	15
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	102	41
Gross derivative instruments not offset in the balance sheet	6	4
Gross cash collateral received not offset in the balance sheet	9	0
Net derivative asset	87	37
Gross derivative liability amount recognized on the balance sheet	82	125
Gross derivative instruments not offset in the balance sheet	6	4
Gross cash collateral posted not offset in the balance sheet	0	0
Net derivative liability	$ 76	$ 121