Retirement Benefits (Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Pension Benefits [Member]
Amounts recognized in regulatory assets, Prior service cost (credit)	$ (1)	[1]
Amounts recognized in regulatory assets, Net actuarial loss	87	[1]
Amounts recognized in accumulated OCI, Prior service cost (credit)	(1)	[1]
Amounts recognized in accumulated OCI, Net actuarial loss	6	[1]
Total	91	[1]	80	[1]	65	[1]	81	[1]
Postretirement Benefits [Member]
Amounts recognized in regulatory assets, Transition obligation	2	[1]
Amounts recognized in regulatory assets, Prior service cost (credit)	(4)	[1]
Amounts recognized in regulatory assets, Net actuarial loss	23	[1]
Amounts recognized in accumulated OCI, Prior service cost (credit)	(1)	[1]
Amounts recognized in accumulated OCI, Net actuarial loss	3	[1]
Total	$ 23	[1]	$ 25	[1]	$ 21	[1]	$ 34	[1]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.