Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation	$ 87	$ 449
Defined Benefit Plan, Settlements Benefit obligation threshold		224
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax		77
Liability, Retirement and Postemployment Benefits	$ 717	$ 717		$ 711	$ 665	$ 699
Defined Benefit Plan, Funded Percentage	80.00%	80.00%
Capitalization of pension and OPEB expense related to engineering and plant construction		$ 20	$ 18
Additional contributions to pension plan		60
Plan assets	$ 2,604	2,604				$ 2,766
Decrease in pension assets		$ 162
Percentage of increase in pension plan		6.00%
Positive investment returns		$ 270
Benefit payments		492
CTF Transaction [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Differental payment received		131
Pension Costs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax		$ 77