Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 24	$ 23
Union Electric Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	16	22
Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	8	1
Affiliates [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1
Affiliates [Member] | Union Electric Company [Member]
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	3	2
Commodity Marketing Companies [Member] | Union Electric Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	2
Commodity Marketing Companies [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	2
Electric Utilities [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	3
Electric Utilities [Member] | Union Electric Company [Member]
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	14	15
Financial Companies [Member] | Union Electric Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	10	14
Financial Companies [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	4	1
Municipalities/Cooperatives [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	3	3
Municipalities/Cooperatives [Member] | Union Electric Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	3	3
Oil And Gas Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	2
Oil And Gas Companies [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 2