Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation		$ 362
Defined Benefit Plan, Settlements Benefit obligation threshold		234
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax		$ 62
Liability, Retirement and Postemployment Benefits				$ 699
Defined Benefit Plan, Funded Percentage		80.00%
Capitalization of pension and OPEB expense related to engineering and plant construction		$ 14	$ 13
Additional contributions to pension plan		41
Plan assets receivable		38
Plan assets		2,609		$ 2,766
Defined Benefit Plan, Fair Value of Plan Assets, Period Increase (Decrease)		$ 157
Percentage of increase in pension plan		6.00%
Positive investment returns		$ 154
Benefit payments		$ 390
CTF Transaction [Member] | Scenario, Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Verizon Differential Payment Expected	$ (131)
Differential Retained For Contribution Made Prior To Receipt	$ 34