Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2017	Dec. 31, 2016
Asset Retirement Obligation [Line Items]
Balance	[1]	$ 649	$ 650
Liabilities settled		(1)
Accretion	[2]	13
Change of estimates	[3]	(13)
Other current liabilities		298	274
Union Electric Company
Asset Retirement Obligation [Line Items]
Balance	[1]	644	644
Liabilities settled		(1)
Accretion	[2]	13
Change of estimates	[3]	(12)
Other current liabilities		130	123
Ameren Illinois Company
Asset Retirement Obligation [Line Items]
Balance	[4]	5	6
Liabilities settled	[4],[5]	(1)
Accretion	[2],[5]	1
Change of estimates	[3],[4]	(1)
Other current liabilities		128	109
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation [Line Items]
Other current liabilities		15	15
Asset Retirement Obligation Balance [Member] | 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 December 31, 2016 and June 30, 2017, respectively.

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

[3] Ameren Missouri changed its fair value estimate primarily related to extending the remediation period of certain CCR storage facilities.

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

[5] Less than $1 million.