Derivative Financial Instruments (Offsetting Assets and Liabilities) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	$ 208	$ 90
Gross derivative instruments not offset in the balance sheet	52	19
Gross cash collateral received not offset in the balance sheet	30	0
Net derivative asset	126	71
Gross derivative liability amount recognized on the balance sheet	236	202
Gross derivative instruments not offset in the balance sheet	52	19
Gross cash collateral posted not offset in the balance sheet	127	47
Net derivative liability	57	136
Union Electric Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	112	49
Gross derivative instruments not offset in the balance sheet	36	15
Gross cash collateral received not offset in the balance sheet	15	0
Net derivative asset	61	34
Gross derivative liability amount recognized on the balance sheet	171	77
Gross derivative instruments not offset in the balance sheet	36	15
Gross cash collateral posted not offset in the balance sheet	127	47
Net derivative liability	8	15
Ameren Illinois Company
Derivative [Line Items]
Gross derivative asset amount recognized on the balance sheet	96	41
Gross derivative instruments not offset in the balance sheet	16	4
Gross cash collateral received not offset in the balance sheet	15	0
Net derivative asset	65	37
Gross derivative liability amount recognized on the balance sheet	65	125
Gross derivative instruments not offset in the balance sheet	16	4
Gross cash collateral posted not offset in the balance sheet	0	0
Net derivative liability	$ 49	$ 121