Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation		$ 589	$ 396
Asset Retirement Obligation, Accretion Expense	[1]	11
Asset Retirement Obligation, Revision of Estimate	[2]	182
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation		582	389
Asset Retirement Obligation, Accretion Expense	[1]	11
Asset Retirement Obligation, Revision of Estimate	[2]	182
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	[3]	7	$ 7
Asset Retirement Obligation, Accretion Expense	[1],[4]	1
Asset Retirement Obligation, Revision of Estimate	[2],[4]	1
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] Accretion expense was recorded as an increase to regulatory assets.

[2] The ARO increase resulted in a corresponding increase recorded to “Property and Plant, Net.” During the first quarter of 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, during the second quarter of 2015, 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. Also during the second quarter of 2015, Ameren and Ameren Missouri 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] Included in “Other deferred credits and liabilities” on the balance sheet

[4] Less than $1 million.