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
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	$ (1)	[1]
Net actuarial loss	97	[1]
Prior service cost (credit)	(2)	[1]
Net actuarial loss	7	[1]
Net periodic benefit cost	101	[1]
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	(4)	[1]
Net actuarial loss	19	[1]
Prior service cost (credit)	(9)	[1]
Net actuarial loss	5	[1]
Net periodic benefit cost	$ 11	[1]

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