Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 192	[1]	$ 226	[1]
Cash Collateral Posted	27		61
Potential Aggregate Amount of Additional Collateral Required	127	[2]	155	[2]
Ameren Missouri [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	76	[1]	78	[1]
Cash Collateral Posted	1		3
Potential Aggregate Amount of Additional Collateral Required	45	[2]	71	[2]
Ameren Illinois Company [Member]
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	116	[1]	148	[1]
Cash Collateral Posted	26		58
Potential Aggregate Amount of Additional Collateral Required	$ 82	[2]	$ 84	[2]

[1] Prior to consideration of master trading and netting agreements and including NPNS 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 netting effects of such agreements.