Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 646	$ 644
Liabilities settled	(4)
Accretion	20
Change of estimates	(14)
Other current liabilities	317	326
Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	6	6
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	642	640
Liabilities settled	(4)
Accretion	20
Change of estimates	(14)
Other current liabilities	115	103
Union Electric Company \| Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	6	6
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	4	4
Liabilities settled	(1)
Accretion	1
Change of estimates	0
Other current liabilities	$ 161	$ 177

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- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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Amount of liabilities classified as other, due within one year or the normal operating cycle, if longer.

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