Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning of period	$ 50	$ 17
Bad debt expense	9	48
Net write-offs	(30)	(15)
End of period	29	50
Union Electric Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning of period	16	7
Bad debt expense	5	15
Net write-offs	(8)	(6)
End of period	13	16
Ameren Illinois Company
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning of period	34	10
Bad debt expense	4	33
Net write-offs	(22)	(9)
End of period	$ 16	$ 34