The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 71

-Paragraph 11

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_RegulatoryLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Rate And Regulatory Matters (Narrative-Illinois) (Detail) (Ameren Illinois Company [Member], USD $) In Millions, unless otherwise specified		12 Months Ended	0 Months Ended
	Dec. 31, 2012 Ieima [Member] Electric Distribution [Member] Final Rate Order [Member]	Dec. 31, 2012 Illinois Commerce Commission [Member]	Apr. 19, 2013 Illinois Commerce Commission [Member] Subsequent Event [Member]	Dec. 31, 2012 Revenue Subject to Refund [Member] Illinois Commerce Commission [Member]
Rate And Regulatory Matters [Line Items]
Revenue requirement	$ 765		$ 785
Requested rate decrease, amount			30
Regulatory liability				50
Approved rate increase, amount		$ 20