Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	$ 19	$ 19	$ 17	$ 18
Bad debt expense	7	5	10	8
Net write-offs	(1)	(5)	(2)	(7)
End of period	25	19	25	19
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	8	7	7	7
Bad debt expense	2	2	4	3
Net write-offs	(1)	(2)	(2)	(3)
End of period	9	7	9	7
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning of period	11	12	10	11
Bad debt expense	5	3	6	5
Net write-offs	0	(3)	0	(4)
End of period	$ 16	$ 12	$ 16	$ 12