Summary Of Significant Accounting Policies (Schedule Of Rates Used For Allowance For Funds Used During Construction) (Details)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Minimum [Member]
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	8.00%	8.00%	8.00%
Maximum [Member]
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	9.00%	9.00%	9.00%
Union Electric Company [Member]
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	8.00%	8.00%	8.00%
Ameren Illinois Company [Member]
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	9.00%	9.00%	9.00%

Disclosure of the effective rate for the period representing (a) total additions to qualifying regulated assets for allowance for funds used during construction, (b) divided by the sum of debt and equity funds used for construction.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 835

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

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Period Type:	duration