Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 16	$ 15
Union Electric Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	12	15
Ameren Illinois Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	4
Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1
Affiliates [Member] | Union Electric Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	3	1
Commodity Marketing Companies [Member] | Union Electric Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1	1
Commodity Marketing Companies [Member] | Ameren Illinois Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2
Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1	1
Electric Utilities [Member] | Union Electric Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1	1
Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	8	10
Financial Companies [Member] | Union Electric Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	6	10
Financial Companies [Member] | Ameren Illinois Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	3	3
Municipalities/Cooperatives [Member] | Union Electric Company [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 3	$ 3