Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (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	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 2,274
Net current-period other comprehensive income (loss)	$ 71	$ (3)	70	$ 58
Balance	2,401		2,401
Pension Costs/Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(345)	(403)
Other comprehensive income before reclassifications, net of tax			45	12
Amounts reclassified from accumulated other comprehensive loss, net of tax			27	50
Net current-period other comprehensive income (loss)			72	62
Balance	(273)	(341)	(273)	(341)
OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(21)	16
Other comprehensive income before reclassifications, net of tax			1	(1)
Amounts reclassified from accumulated other comprehensive loss, net of tax			(3)	(3)
Net current-period other comprehensive income (loss)			(2)	(4)
Balance	(23)	12	(23)	12
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	195	194	195	194	$ 223	$ 231
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(366)	(387)
Other comprehensive income before reclassifications, net of tax			46	11
Amounts reclassified from accumulated other comprehensive loss, net of tax			24	47
Net current-period other comprehensive income (loss)			70	58
Balance	$ (296)	$ (329)	$ (296)	$ (329)