Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 170	[1]
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	159	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	96	[1]
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	88	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	74	[1]
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 71	[2]

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

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