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, 2013
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	$ (1)	[1]
Net actuarial loss	60	[1]
Net actuarial (gain) loss	1	[1]
Net periodic benefit cost	60	[1]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)	(4)	[1]
Net actuarial loss	9	[1]
Net actuarial (gain) loss	(2)	[1]
Net periodic benefit cost	$ 3	[1]

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