Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($)	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 7,000,000	$ 8,000,000
Restricted cash included in “Other current assets”	10,000,000	16,000,000
Restricted cash included in “Other assets”	132,000,000	127,000,000
Restricted cash included in “Nuclear decommissioning trust fund”	5,000,000	4,000,000
Total cash, cash equivalents, and restricted cash	154,000,000	155,000,000	$ 172,000,000	$ 301,000,000
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	1,000,000	0
Restricted cash included in “Other current assets”	3,000,000	4,000,000
Restricted cash included in “Other assets”	0	0
Restricted cash included in “Nuclear decommissioning trust fund”	5,000,000	4,000,000
Total cash, cash equivalents, and restricted cash	9,000,000	8,000,000	12,000,000	145,000,000
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0
Restricted cash included in “Other current assets”	5,000,000	6,000,000
Restricted cash included in “Other assets”	132,000,000	127,000,000
Restricted cash included in “Nuclear decommissioning trust fund”	0	0
Total cash, cash equivalents, and restricted cash	$ 137,000,000	$ 133,000,000	$ 149,000,000	$ 147,000,000