Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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)	$ (34)	$ (20)	36	$ 38
Balance	1,943		1,943
Pension Costs/Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(345)	(403)
Other comprehensive income before reclassifications, net of tax			1	(21)
Amounts reclassified from accumulated other comprehensive loss, net of tax			38	63
Net current-period other comprehensive income (loss)			39	42
Balance	(306)	(361)	(306)	(361)
OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(21)	16
Other comprehensive income before reclassifications, net of tax			1
Amounts reclassified from accumulated other comprehensive loss, net of tax			(4)	(4)
Net current-period other comprehensive income (loss)			(3)	(4)
Balance	(24)	12	(24)	12
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	206	209	206	209	$ 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			2	(21)
Amounts reclassified from accumulated other comprehensive loss, net of tax			34	59
Net current-period other comprehensive income (loss)			36	38
Balance	$ (330)	$ (349)	$ (330)	$ (349)