Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation		$ 255
Defined Benefit Plan, Settlements Benefit obligation threshold		234
Pension settlement costs		43
Liability, Retirement and Postemployment Benefits		655		$ 699
Capitalization of pension and OPEB expense related to engineering and plant construction	$ 7	7
Defined benefit plan, cash contributions by employer	31
Plan assets	2,558		$ 2,674
Decrease in pension assets	$ 116
Percentage of increase in pension plan	4.00%
Negative investment returns	$ (45)
Benefit payments	$ 102
Pension Costs/Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension settlement costs		$ 43