Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Asset Retirement Obligation [Line Items]
Balance	[1]	$ 646	$ 650
Liabilities settled		(5)
Accretion	[2]	20
Change of estimates	[3]	(19)
Other current liabilities		318	274
Union Electric Company
Asset Retirement Obligation [Line Items]
Balance	[1]	642	644
Liabilities settled		(4)
Accretion	[2]	20
Change of estimates	[3]	(18)
Other current liabilities		118	123
Ameren Illinois Company
Asset Retirement Obligation [Line Items]
Balance	[4]	4	6
Liabilities settled		(1)
Accretion	[2],[5]	1
Change of estimates	[3],[4]	(1)
Other current liabilities		153	109
Asset Retirement Obligation Balance
Asset Retirement Obligation [Line Items]
Other current liabilities		15	15
Asset Retirement Obligation Balance | Union Electric Company
Asset Retirement Obligation [Line Items]
Other current liabilities		$ 15	$ 15

[1] Balance included $15 million in “Other current liabilities” on the balance sheet as of both December 31, 2016 and September 30, 2017, respectively.

[2] Accretion expense was recorded as a decrease to regulatory liabilities.

[3] Ameren Missouri changed its fair value estimate primarily due to an extension of the remediation period of certain CCR storage facilities, an update to the decommissioning of the Callaway energy center to reflect the cost study and funding analysis filed with the MoPSC in 2017, and an increase in the discount rate assumption.

[4] Included in “Other deferred credits and liabilities” on the balance sheet.

[5] Less than $1 million.