Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 4,519
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	77
Balance	3,410
Pension Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(647)	$ (584)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	(34)	(105)
Amounts reclassified from accumulated other comprehensive loss, before tax	23	30
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	77
Net current-period other comprehensive income (loss), before tax	66	(75)
Balance	(581)	(659)
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	29	20
Other Comprehensive Income (Loss), before Reclassifications, before Tax	1
Amounts reclassified from accumulated other comprehensive loss, before tax	(7)	(6)
Net current-period other comprehensive income (loss), before tax	(6)	(6)
Balance	23	14
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	231	211
Other Comprehensive Income (Loss) before Reclassifications, Tax	12	40
Amounts reclassified from AOCI, tax	(5)	(9)
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Tax	(29)
Net current-period other comprehensive income (loss), tax	(22)	31
Balance	209	242
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(387)	(353)
Other Comprehensive income (loss), before reclassifications, Net of Tax	(21)	(65)
Amounts reclassified from accumulated other comprehensive loss to net loss, net of tax	11	15
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax	48
Other Comprehensive Income (loss) and Settlement Costs, Net of Tax	38	(50)
Balance	(349)	$ (403)
Accumulated Other Comprehensive Loss [Member] | Pension Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	77
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax	$ 48