Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 170	[1]
Cash Collateral Posted	8
Potential Aggregate Amount of Additional Collateral Required	150	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	88	[1]
Cash Collateral Posted	7
Potential Aggregate Amount of Additional Collateral Required	75	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	82	[1]
Cash Collateral Posted	1
Potential Aggregate Amount of Additional Collateral Required	$ 75	[2]

[1] Prior to consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures.

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