Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Mar. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Settlements, Benefit Obligation			$ 171	$ 362
Defined Benefit Plan, Settlements Benefit obligation threshold			216	234
Pension settlement costs	$ 25	$ 19	25	62
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax		19	25	62
Liability, Retirement and Postemployment Benefits	570	711	570	711	$ 689	$ 665
Capitalization of pension and OPEB expense related to engineering and plant construction			14	14
Defined benefit plan, cash contributions by employer			64
Plan assets	2,499		2,499		$ 2,674
Decrease in pension assets			$ 175
Percentage of increase in pension plan			7.00%
Negative investment returns			$ (37)
Benefit payments			202
Pension Costs/Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension settlement costs	25	$ 19	25	$ 62
Plan contributions required, 2018	$ 150		150
Plan contributions			$ 64