Fair Value Measurements (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	$ 1	$ 7
Union Electric Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	1	1
Ameren Illinois Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	$ 1	$ 7