Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 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	62
Balance	3,566
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(387)
Balance	(329)
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	62
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax	36
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(387)	$ (353)
Other Comprehensive income (loss), before reclassifications, Net of Tax	11
Amounts reclassified from accumulated other comprehensive loss, net of tax	11	11
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax	36
Other Comprehensive Income (loss) and Settlement Costs, Net of Tax	58	11
Balance	(329)	(342)
Pension Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(647)	(584)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	25
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(17)	(21)
Net current-period other comprehensive income (loss), before tax	104	21
Balance	(543)	(563)
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	29	20
Other Comprehensive Income (Loss), before Reclassifications, before Tax	(5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	4	4
Net current-period other comprehensive income (loss), before tax	(9)	(4)
Balance	20	16
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	(9)
Amounts reclassified from AOCI, tax	(2)	(6)
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Tax	(26)
Net current-period other comprehensive income (loss), tax	(37)	(6)
Balance	$ 194	$ 205