Rate And Regulatory Matters (Narrative-Illinois) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Mar. 31, 2012 IEIMA [Member] Ameren Illinois Company [Member]	Apr. 30, 2012 Electric Distribution [Member] IEIMA [Member] Initial Filing [Member] Ameren Illinois Company [Member]	Apr. 20, 2012 Electric Distribution [Member] IEIMA [Member] Update Filing [Member] Ameren Illinois Company [Member]	Jan. 03, 2012 ICC Staff Recommendation [Member] Electric Distribution [Member] IEIMA [Member] Initial Filing [Member] Ameren Illinois Company [Member]
Utility revenue decrease requested						$ 19	$ 15	$ 25
Regulatory assets	$ 1,657	$ 1,603	$ 814	$ 748	$ 12

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 71

-Paragraph 9, 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

Name:	us-gaap_RegulatoryAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant