Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 123	[1]	$ 145	[1]
Cash Collateral Posted	2		17
Potential Aggregate Amount of Additional Collateral Required	113	[2]	122	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	62	[1]	70	[1]
Cash Collateral Posted	2		2
Potential Aggregate Amount of Additional Collateral Required	57	[2]	67	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	61	[1]	75	[1]
Cash Collateral Posted			15
Potential Aggregate Amount of Additional Collateral Required	$ 56	[2]	$ 55	[2]

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