Retirement Plans (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Apr. 01, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate - used at year end to value obligation (in hundredths)		4.50%		4.00%
Expected long-term rate of return on plan assets (in hundredths)	7.50%	7.75%	7.75%
Expected Long Term Rate Of Return On Plan Assets In Future Year In Hundreths	7.50
Expected amortization of unrecognized loss	$ 35
Capitalization of pension and OPEB expense related to engineering and plant construction	25	$ 20	$ 15
Pension/OPEB costs	79	10	(18)
Contribution of Property	$ 15		15
Defined Benefit Plan, Lease term of contributed property	15 years
Pension Building Contribution Aggregate Annual Rent	$ 2
Assets transferred to plan as a result of acquisition	1,108
Accumulated benefit obligation	3,363	2,048
401(k) savings plan employer contributions	$ 48	$ 28	$ 21
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)	4.10%	4.50%	4.10%
Expected long-term rate of return on plan assets (in hundredths)	7.50%	7.75%	7.75%
Contributions to plans	$ 28	$ 62	$ 83
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	0
Contributions to plans	$ 18	$ 20
Annual rate of increase in the per-capita cost of covered medical benefits (in hundredths)	7.00%
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	20
Effect on total of service and interest cost components, 1 percentage point decrease	(1)
Effect on postretirement benefit obligation, 1 percentage point decrease	$ (19)
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)	4.10%	4.50%	4.10%
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)	4.30%	4.70%	4.20%
2016 Pension Real Estate Contributions [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contribution of Property	$ 15
Cash [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer Cash Contributions	$ 13
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%