Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 2,274
Balance	2,396
Pension Costs/Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(345)	$ (403)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	(4)	30
Amounts reclassified from accumulated other comprehensive loss, before tax	4	32
Net current-period other comprehensive income (loss), before tax		62
Balance	(345)	(341)
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(21)	16
Other Comprehensive Income (Loss), before Reclassifications, before Tax
Amounts reclassified from accumulated other comprehensive loss, before tax	(1)	(1)
Net current-period other comprehensive income (loss), before tax	(1)	(1)
Balance	(22)	15
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	218	195	$ 223	$ 231
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(366)	(387)
Other Comprehensive income (loss), before reclassifications, Net of Tax	(4)	30
Amounts reclassified from accumulated other comprehensive loss to net loss, net of tax	3	31
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax		61
Other Comprehensive Income (loss) and Settlement Costs, Net of Tax	(1)
Balance	$ (367)	(326)
Accumulated Other Comprehensive Loss [Member] | Pension Costs/Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax		$ 43