Callaway Energy Center (Narrative) (Detail) (Nuclear Plant [Member], USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013 mill	Dec. 31, 2012	Dec. 31, 2011
Nuclear Plant [Member]
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee	1
Settlement payment	$ 6
Assumed life of plant, in years	40 years
Annual decommissioning costs included in costs of service	$ 7	$ 7	$ 7
Missouri Public Service Commission, Requirement to file updated cost study and funding analysis for decommissioning energy center, Period	3 years

Amount of anticipated decommissioning costs included in annual costs of service used to establish electric rates.

No definition available.

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- Definition

Missouri Public Service Commission, Requirement to File Updated Cost Study and Funding Analysis for Decommissioning Energy Center, Period

+ References

No definition available.

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- Definition

The number of mills per nuclear-generated kilowatt hour sold (one mill = 1/10 of one cent) charged by the Department of Energy for future disposal of spent nuclear fuel, referred to as the NWF fee. This fee is collected from electric customers who use power generated by the Callaway nuclear plant.

+ References

No definition available.

+ Details

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- Definition

Amount of gain recognized in the period that had been disclosed in prior periods as a contingent gain.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 30

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6397426&loc=d3e17499-108355

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 17

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

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