Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments)) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	$ 92	$ 90
Union Electric Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	32	31
Ameren Illinois Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	$ (1)	$ (1)

Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

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

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