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