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Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail)
$ in Millions
Sep. 30, 2017
USD ($)
Derivative [Line Items]  
Aggregate Fair Value of Derivative Liabilities $ 107 [1]
Cash Collateral Posted 3
Potential Aggregate Amount of Additional Collateral Required 89 [2]
Union Electric Company  
Derivative [Line Items]  
Aggregate Fair Value of Derivative Liabilities 59 [1]
Cash Collateral Posted 3
Potential Aggregate Amount of Additional Collateral Required 48 [2]
Ameren Illinois Company  
Derivative [Line Items]  
Aggregate Fair Value of Derivative Liabilities 48 [1]
Cash Collateral Posted 0
Potential Aggregate Amount of Additional Collateral Required $ 41 [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.