Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 11	$ 23
Union Electric Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	11	22
Ameren Illinois Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts		1
Commodity Marketing Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	2
Commodity Marketing Companies [Member] | Union Electric Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	2
Electric Utilities [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	3
Electric Utilities [Member] | Union Electric Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	3
Financial Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	7	15
Financial Companies [Member] | Union Electric Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	7	14
Financial Companies [Member] | Ameren Illinois Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts		1
Municipalities/Cooperatives [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	2	3
Municipalities/Cooperatives [Member] | Union Electric Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 2	$ 3