Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 5	$ 15
Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	5	15
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1	1
Commodity Marketing Companies [Member] | Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1	1
Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts		1
Electric Utilities [Member] | Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts		1
Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2	10
Financial Companies [Member] | Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2	10
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2	3
Municipalities/Cooperatives [Member] | Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 2	$ 3