Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 167	$ 90
Gross derivative instruments not offset in the balance sheet	31	19
Gross cash collateral received not offset in the balance sheet	3	0
Net derivative asset	133	71
Gross derivative liability amount recognized on the balance sheet	177	202
Gross derivative instruments not offset in the balance sheet	31	19
Gross cash collateral posted not offset in the balance sheet	99	47
Net derivative liability	47	136
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	73	49
Gross derivative instruments not offset in the balance sheet	18	15
Gross cash collateral received not offset in the balance sheet	2	0
Net derivative asset	53	34
Gross derivative liability amount recognized on the balance sheet	129	77
Gross derivative instruments not offset in the balance sheet	18	15
Gross cash collateral posted not offset in the balance sheet	99	47
Net derivative liability	12	15
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	94	41
Gross derivative instruments not offset in the balance sheet	13	4
Gross cash collateral received not offset in the balance sheet	1	0
Net derivative asset	80	37
Gross derivative liability amount recognized on the balance sheet	48	125
Gross derivative instruments not offset in the balance sheet	13	4
Gross cash collateral posted not offset in the balance sheet	0	0
Net derivative liability	$ 35	$ 121