Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	$ 25	$ 19	$ 17	$ 18
Bad debt expense	21	14	31	22
Net write-offs	(2)	(14)	(4)	(21)
End of period	44	19	44	19
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	9	7	7	7
Bad debt expense	5	3	9	6
Net write-offs	0	(2)	(2)	(5)
End of period	14	8	14	8
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	16	12	10	11
Bad debt expense	16	11	22	16
Net write-offs	(2)	(12)	(2)	(16)
End of period	$ 30	$ 11	$ 30	$ 11