Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2012	Dec. 31, 2011
Fair Value Measurements [Abstract]
Gain (loss) recognized related to valuation adjustments for counterparty default risk	$ (2)
Valuation adjustments related to net derivative contracts	$ 8	$ 1

Gain Loss Recognized Related To Valuation Adjustments For Counterparty Default Risk

No definition available.

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Namespace Prefix:	aee_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration