Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		$ 396,000,000		$ 369,000,000
Liabilities incurred		3,000,000		2,000,000
Liabilities settled		(2,000,000)		(2,000,000)
Accretion in period	[1]	23,000,000		21,000,000
Change in estimates		203,000,000	[2]	6,000,000	[3]
Balance		623,000,000	[4]	396,000,000
Other current liabilities		379,000,000		434,000,000
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		389,000,000		366,000,000
Liabilities incurred		3,000,000		2,000,000
Liabilities settled		(1,000,000)		(2,000,000)
Accretion in period	[1]	23,000,000		21,000,000
Change in estimates		203,000,000	[2]	2,000,000	[3]
Balance		617,000,000	[4]	389,000,000
Other current liabilities		120,000,000		117,000,000
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		7,000,000	[5]	3,000,000
Liabilities incurred		0		0
Liabilities settled		(1,000,000)		(1,000,000)	[6]
Accretion in period	[1],[6]	1,000,000		1,000,000
Change in estimates		1,000,000	[2],[6]	4,000,000	[3]
Balance	[5]	6,000,000		7,000,000
Other current liabilities		114,000,000		$ 124,000,000
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		$ 5,000,000

[1] (b)Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

[2] The ARO increase resulted in a corresponding increase recorded to "Property and Plant, 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.

[3] c)The ARO increase resulted in a corresponding increase recorded to "Property and Plant, Net." Ameren Illinois changed its fair value estimate for asbestos removal.

[4] Balance included $5 million in "Other current liabilities" on the balance sheet as of December 31, 2015

[5] ncluded in “Other deferred credits and liabilities” on the balance sheet.

[6] (a)Less than $1 million.