Fair Value Measurements (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Fair Value Disclosures [Line Items]
Gain (loss) recognized related to valuation adjustments for counterparty default risk	$ (2)	$ 0
Valuation adjustments related to net derivative contracts, liabilities	7		1
Ameren Missouri [Member]
Fair Value Disclosures [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	1		1
Ameren Illinois Company [Member]
Fair Value Disclosures [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	14		19
Genco [Member]
Fair Value Disclosures [Line Items]
Gain (loss) recognized related to valuation adjustments for counterparty default risk	1	(1)
Valuation adjustments related to net derivative contracts, liabilities	1
Valuation adjustments related to net derivative contracts, assets			$ (1)