Derivative Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Cash collateral held from counterparties		$ 1
Counterparty letters of credit held as collateral	7	9
Ameren Missouri [Member]
Derivative [Line Items]
Cash collateral held from counterparties	2
Marketing Company [Member]
Derivative [Line Items]
Cash collateral held from counterparties	$ 4

The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6935481&loc=d3e41678-113959

-Subparagraph (b)

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