Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 170	[1]	$ 226	[1]
Cash Collateral Posted	25		61
Potential Aggregate Amount of Additional Collateral Required	133	[2]	155	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	69	[1]	78	[1]
Cash Collateral Posted	1		3
Potential Aggregate Amount of Additional Collateral Required	63	[2]	71	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	101	[1]	148	[1]
Cash Collateral Posted	24		58
Potential Aggregate Amount of Additional Collateral Required	$ 70	[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 netting effects of such agreements.