Derivative Financial Instruments (Narrative) (Detail) (Union Electric Company, USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Union Electric Company
Derivative [Line Items]
Counterparty letters of credit held as collateral	$ 1

Counterparty letters of credit held as collateral.

No definition available.

Name:	aee_LettersOfCreditHeldAsCollateral
Namespace Prefix:	aee_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant