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, 2017 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ (1)
Net actuarial (gain) loss	60
Net actuarial loss	5
Net periodic benefit cost	64
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(2)
Net actuarial (gain) loss	(1)
Net actuarial loss	0
Net periodic benefit cost	$ (3)