Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		$ 623,000,000	[1]	$ 396,000,000
Liabilities incurred		3,000,000		3,000,000
Liabilities settled		(2,000,000)		(2,000,000)
Accretion in period	[2]	25,000,000		23,000,000
Change in estimates		1,000,000		203,000,000	[3]
Balance		650,000,000	[1]	623,000,000	[1]	$ 396,000,000
Other current liabilities		248,000,000		279,000,000
Callaway energy center decommissioning study [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Change in estimates		99,000,000
New CCR Rules Estimate [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Change in estimates		100,000,000
Other Estimate Changes [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Change in estimates		4,000,000
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities		15,000,000		5,000,000
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		617,000,000	[1]	389,000,000
Liabilities incurred		3,000,000		3,000,000
Liabilities settled		(2,000,000)		(1,000,000)
Accretion in period	[2]	25,000,000		23,000,000
Change in estimates		1,000,000		203,000,000	[3]
Balance		644,000,000	[1]	617,000,000	[1]	389,000,000
Other current liabilities		123,000,000		120,000,000
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		6,000,000	[4]	7,000,000
Liabilities incurred		0		0
Liabilities settled		(1,000,000)	[5]	(1,000,000)
Accretion in period	[2],[5]	1,000,000		1,000,000
Change in estimates		0		1,000,000	[3],[5]
Balance		6,000,000	[4]	6,000,000	[4]	7,000,000
Other current liabilities		94,000,000		86,000,000
Asset Retirement Obligation | Union Electric Company
Asset Retirement Obligation [Line Items]
Noncash Depreciation related to ARO		$ 31,000,000		$ 13,000,000		$ 1,000,000

[1] (e)Balance included $5 million and $15 million in "Other current liabilities" on the balance sheet as of December 31, 2015 and 2016, respectively.

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

[3] (c)The ARO increase resulted in a corresponding increase recorded to "Property, Plant, and Equipment, Net." Ameren and Ameren Missouri increased their AROs related to the decommissioning of the Callaway energy center by $99 million to reflect the 2015 cost study and funding analysis filed with the MoPSC, the extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 – Callaway Energy Center for additional information. In addition, as a result of new federal regulations, Ameren and Ameren Missouri recorded an increase of $100 million to their AROs associated with CCR storage facilities. See Note 15 – Commitments and Contingencies for additional information. Ameren and Ameren Missouri also increased their AROs by $4 million due to a change in the estimated retirement dates of the Meramec and Rush Island energy centers as a result of the MoPSC's April 2015 electric rate order.

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

[5] (b)Less than $1 million.