Supplemental Information (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 17	$ 25
Restricted cash included in “Other current assets”	15	13
Restricted cash included in “Other assets”	287	229
Restricted cash included in “Nuclear decommissioning trust fund”	5	5
Total cash, cash equivalents, and restricted cash	324	272	$ 246	$ 216
Union Electric 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	5
Restricted cash included in “Other assets”	0	0
Restricted cash included in “Nuclear decommissioning trust fund”	5	5
Total cash, cash equivalents, and restricted cash	10	10	13	13
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”	6	5
Restricted cash included in “Other assets”	287	229
Restricted cash included in “Nuclear decommissioning trust fund”	0	0
Total cash, cash equivalents, and restricted cash	$ 293	$ 234	$ 225	$ 191