Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 454	[1]	$ 456	[1]
Cash Collateral Posted	111		116
Potential Aggregate Amount of Additional Collateral Required	335	[2]	332	[2]
Ameren Missouri [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	146	[1]	102	[1]
Cash Collateral Posted	7		8
Potential Aggregate Amount of Additional Collateral Required	125	[2]	86	[2]
Ameren Illinois Company [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	174	[1]	220	[1]
Cash Collateral Posted	91		96
Potential Aggregate Amount of Additional Collateral Required	106	[2]	125	[2]
Genco [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	48	[1]	55	[1]
Cash Collateral Posted	1		1
Potential Aggregate Amount of Additional Collateral Required	$ 41	[2]	$ 58	[2]

[1] Prior to consideration of master trading and netting agreements and including NPNS contract exposures.

[2] As collateral requirements with certain counterparties are based on master trading and netting agreements, the aggregate amount of additional collateral required to be posted is after consideration of the effects of such agreements.