Retirement Benefits (Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	[1]
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ (1)
Net actuarial (gain) loss	50
Net actuarial loss	4
Net periodic benefit cost	53
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(5)
Net actuarial (gain) loss	(7)
Net actuarial loss	0
Net periodic benefit cost	$ (12)

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