Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	$ 47	$ 19	$ 50	$ 17
Bad debt expense	(3)	7	1	10
Net write-offs	(2)	(1)	(9)	(2)
End of period	42	25	42	25
Payables for purchased receivables	30		30		$ 28
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	15	8	16	7
Bad debt expense	2	2	3	4
Net write-offs	(1)	(1)	(3)	(2)
End of period	16	9	16	9
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	32	11	34	10
Bad debt expense	(5)	5	(2)	6
Net write-offs	(1)	0	(6)	0
End of period	26	$ 16	26	$ 16
Payables for purchased receivables	$ 30		$ 30		$ 28