Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension settlement costs	$ 83
Capitalization of pension and OPEB expense related to engineering and plant construction	$ 26	$ 25	$ 20
Contribution of Property		15
Defined Benefit Plan, Lease term of contributed property	15 years
Pension Building Contribution Aggregate Annual Rent		2
Plan assets acquired during the period		1,108
Accumulated benefit obligation	$ 3,363	3,465
401(k) savings plan employer contributions	$ 48	$ 48	$ 28
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate - used at year end to value obligation (in hundredths)	3.70%	4.10%	4.50%	4.00%
Other comprehensive income (loss) before reclassifications		$ (105)
Expected long-term rate of return on plan assets (in hundredths)	7.50%	7.50%	7.75%
Expected Long Term Rate Of Return On Plan Assets In Future Year In Hundreths	7.50%
Expected amortization of unrecognized loss	$ 26
Defined Benefit Plan, Settlements, Benefit Obligation	486
Defined Benefit Plan, Settlements Benefit obligation threshold	224
Pension settlement costs	83
Contributions to plans	206	$ 28
Contributions to plan, net of differential	$ 75
Employer Cash Contributions		13	$ 62
Defined Benefit Plan, Funded Percentage	80.00%
Differential Payment Received	$ 131
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected amortization of prior service credit	9
Expected amortization of unrecognized loss	2
Contributions to plans	$ 24	$ 18
Annual rate of increase in the per-capita cost of covered medical benefits (in hundredths)	6.75%
Annual rate of increase in the per-capita cost of covered medical benefits in 2024 (in hundredths)	5.00%
Effect on total of service and interest cost components, 1 percentage point increase	$ 1
Effect on postretirement benefit obligation, 1 percentage point increase	22
Effect on total of service and interest cost components, 1 percentage point decrease	(1)
Effect on postretirement benefit obligation, 1 percentage point decrease	$ (22)
Postretirement Benefits Other Than Pensions (OPEB) [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate - used at year end to value obligation (in hundredths)	3.70%	4.10%	4.50%
Postretirement Benefits Other Than Pensions (OPEB) [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate - used at year end to value obligation (in hundredths)	3.80%	4.30%	4.70%
2016 Pension Real Estate Contributions [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution of Property		$ 15
Fixed Income Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation	40.00%
Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation	60.00%