Comprehensive Loss (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	$ 4,519	$ 5,614	$ 3,658
Net current-period other comprehensive income (loss)	21	(34)	51
Balance, ending	2,274	4,519	5,614
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	223	231	211	$ 247
Defined Benefit Plans [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	(403)	(363)	(330)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(12)	(65)	(51)
Amounts reclassified from accumulated other comprehensive loss, net of tax	70	25	18
Net current-period other comprehensive income (loss)	58	(40)	(33)
Balance, ending	(345)	(403)	(363)
Defined Benefit Plans [Member] | Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	16	10	(74)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(31)	11	82
Amounts reclassified from accumulated other comprehensive loss, net of tax	(6)	(5)	2
Net current-period other comprehensive income (loss)	(37)	6	84
Balance, ending	(21)	16	10
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	(387)	(353)	(404)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(43)	(54)	31
Amounts reclassified from accumulated other comprehensive loss, net of tax	64	20	20
Net current-period other comprehensive income (loss)	21	(34)	51
Balance, ending	$ (366)	$ (387)	$ (353)