Commitments And Contingencies (Other Obligations) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Commitments And Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligation	$ 7,515
Unrecognized tax benefits	190
Union Electric Company [Member]
Commitments And Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligation	5,151
Unrecognized tax benefits	126
Ameren Illinois Company [Member]
Commitments And Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligation	2,316
Unrecognized tax benefits	$ 4

The gross amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns as of the balance sheet date.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Unrecognized Tax Benefit

-URI http://asc.fasb.org/extlink&oid=6527854

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15A

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Interpretation (FIN)

-Number 48

-Paragraph 21

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Name:	us-gaap_UnrecognizedTaxBenefits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant