Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 145	[1]	$ 226	[1]
Cash Collateral Posted	17		61
Potential Aggregate Amount of Additional Collateral Required	122	[2]	155	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	70	[1]	78	[1]
Cash Collateral Posted	2		3
Potential Aggregate Amount of Additional Collateral Required	67	[2]	71	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	75	[1]	148	[1]
Cash Collateral Posted	15		58
Potential Aggregate Amount of Additional Collateral Required	$ 55	[2]	$ 84	[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.