Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 31	$ 23
Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	29	22
Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	2	1
Commodity Marketing Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	3	2
Commodity Marketing Companies [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	3	2
Electric Utilities [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	5	3
Electric Utilities [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	5	3
Financial Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	17	15
Financial Companies [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	16	14
Financial Companies [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1	1
Municipalities/Cooperatives [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	5	3
Municipalities/Cooperatives [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	5	3
Oil And Gas Companies [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	1
Oil And Gas Companies [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 1