Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation		$ 597	$ 396
Asset Retirement Obligation, Liabilities Incurred	[1]	3
Asset Retirement Obligation, Liabilities Settled		(1)
Asset Retirement Obligation, Accretion Expense	[2]	17
Asset Retirement Obligation, Revision of Estimate	[3]	182
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	[4]	7	7
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Liabilities Settled	[5]	1
Asset Retirement Obligation, Accretion Expense	[2],[5]	1
Asset Retirement Obligation, Revision of Estimate	[5]	1
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation		590	$ 389
Asset Retirement Obligation, Liabilities Incurred	[1]	3
Asset Retirement Obligation, Liabilities Settled		(1)
Asset Retirement Obligation, Accretion Expense	[2]	17
Asset Retirement Obligation, Revision of Estimate	[3]	182
Callaway energy center decommissioning study
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		99
Callaway energy center decommissioning study | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		99
New CCR Rules Estimate
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		79
New CCR Rules Estimate | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		79
Other Estimate Changes
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		4
Other Estimate Changes | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		$ 4

[1] Ameren and Ameren Missouri recorded a new ARO of $3 million related to the Callaway energy center’s dry spent fuel storage facility. See Note 10 - Callaway Energy Center for additional information.

[2] Accretion expense was recorded as an increase to regulatory assets.

[3] The ARO increase resulted in a corresponding increase recorded to “Property and Plant, Net.” During 2015, 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, 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 $79 million to their AROs associated with CCR storage facilities. See Note 9 - 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] Included in “Other deferred credits and liabilities” on the balance sheet.

[5] Less than $1 million.