Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	$ 29	$ 50
Bad debt expense	4	4
Net write-offs	(5)	(7)
End of period	28	47
Payables for purchased receivables	29		$ 27
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	13	16
Bad debt expense	1	1
Net write-offs	(3)	(2)
End of period	11	15
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	16	34
Bad debt expense	3	3
Net write-offs	(2)	(5)
End of period	17	$ 32
Payables for purchased receivables	$ 29		$ 27