Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 356	[1]	$ 456	[1]
Cash Collateral Posted	68		116
Potential Aggregate Amount of Additional Collateral Required	245	[2]	332	[2]
Ameren Missouri [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	78	[1]	102	[1]
Cash Collateral Posted	3		8
Potential Aggregate Amount of Additional Collateral Required	71	[2]	86	[2]
Ameren Illinois [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	148	[1]	220	[1]
Cash Collateral Posted	58		96
Potential Aggregate Amount of Additional Collateral Required	84	[2]	125	[2]
Other [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	130	[1],[3]	134	[1],[3]
Cash Collateral Posted	7	[3]	12	[3]
Potential Aggregate Amount of Additional Collateral Required	$ 90	[2],[3]	$ 121	[2],[3]

[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.

[3] Includes amounts for Marketing Company, Genco, and Ameren (parent).