Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 187	[1]	$ 226	[1]
Cash Collateral Posted	32		61
Potential Aggregate Amount of Additional Collateral Required	140	[2]	155	[2]
Union Electric Company [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	66	[1]	78	[1]
Cash Collateral Posted	1		3
Potential Aggregate Amount of Additional Collateral Required	62	[2]	71	[2]
Ameren Illinois Company [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	121	[1]	148	[1]
Cash Collateral Posted	31		58
Potential Aggregate Amount of Additional Collateral Required	$ 78	[2]	$ 84	[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 determined after consideration of the effects of such agreements.