Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Mar. 31, 2017 USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 96	[1]
Cash Collateral Posted	8
Potential Aggregate Amount of Additional Collateral Required	74	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	58	[1]
Cash Collateral Posted	8
Potential Aggregate Amount of Additional Collateral Required	44	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	38	[1]
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 30	[2]

[1] Before 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.