Supplemental Information Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 17	$ 18
Bad debt expense	3	3
Net write-offs	(1)	(2)
Ending balance	19	19
Union Electric Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	7	7
Bad debt expense	2	1
Net write-offs	(1)	(1)
Ending balance	8	7
Ameren Illinois Company
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	10	11
Bad debt expense	1	2
Net write-offs	0	(1)
Ending balance	$ 11	$ 12