Retirement Plans (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) property	Dec. 31, 2013 USD ($) property
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted average asset allocation (in hundreths)	100.00%	100.00%
Expected Long Term Rate Of Return On Plan Assets In Future Year In Hundreths	7.50
Pension settlement costs			$ 44
Capitalization of pension and OPEB expense related to engineering and plant construction	$ 20	$ 15	19
Pension/OPEB costs	10	(18)	37
Contribution of Property			23
Plan assets	1,572	1,673
Accumulated benefit obligation	2,048	2,094
401(k) savings plan employer contributions	$ 28	$ 21	$ 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)	4.50%	4.10%	4.90%
Expected long-term rate of return on plan assets (in hundredths)	7.75%	7.75%	8.00%
Expected amortization of unrecognized loss	$ 42
Defined Benefit Plan, Settlements, Benefit Obligation		$ 165
Defined Benefit Plan, Settlements Benefit obligation threshold		$ 125
Pension settlement costs			$ 44
Defined Benefit Plan, Number of properties contributed | property		4
Contribution of Property		$ 23
Defined Benefit Plan, Lease term of contributed property		15 years
Contributions to plans	62	$ 83	62
Plan assets	1,572	1,673	1,217
Assets transferred to plan as a result of acquisition	5	342
Defined benefit plan, cash contributions by employer			39
Benefit payments	128	110
Increase (Decrease) in Pension and Postretirement Obligations	24
Pension Benefits [Member] | Receivable [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Assets transferred to plan as a result of acquisition		34
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	20	12
Plan assets			$ 2
Benefit payments	$ 25	$ 19
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	$ 2
Effect on postretirement benefit obligation, 1 percentage point increase	18
Effect on total of service and interest cost components, 1 percentage point decrease	(2)
Effect on postretirement benefit obligation, 1 percentage point decrease	(16)
Increase (Decrease) in Pension and Postretirement Obligations	$ 66
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)	4.50%	4.10%	4.90%
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)	4.70%	4.20%	5.20%
2013 Pension Real Estate Contributions [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Number of properties contributed | property			4
Defined Benefit Plan, Lease term of contributed property	15 years
Pension Building Contribution Aggregate Annual Rent	$ 2
Common Stock Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted average asset allocation (in hundreths)	47.00%	45.00%
Common Stock Securities [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted average asset allocation (in hundreths)	40.00%
Alternative Investments [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted average asset allocation (in hundreths)	6.00%	6.00%
Debt Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted average asset allocation (in hundreths)	46.00%	47.00%
Debt Securities [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted average asset allocation (in hundreths)	60.00%