Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	43
Balance	4,326
Defined Benefit Plans [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(647)	$ (584)
Other Comprehensive Income (Loss), before Reclassifications, before Tax	48
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(8)	(11)
Other comprehensive income, Recognition of net actuarial loss for pension settlement costs, before tax	43
Net current-period other comprehensive income (loss), before tax	99	11
Balance	(548)	(573)
Defined Benefit Plans [Member] | Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	29	20
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	2	2
Net current-period other comprehensive income (loss), before tax	(2)	(2)
Balance	27	18
Defined Benefit Plans [Member] | 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	(19)
Amounts reclassified from AOCI, tax	1	(3)
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Tax	(18)
Net current-period other comprehensive income (loss), tax	(36)	(3)
Balance	195	208
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(387)	(353)
Other Comprehensive income (loss), before reclassifications, Net of Tax	29
Amounts reclassified from accumulated other comprehensive loss, net of tax	7	6
Other Comprehensive Income, Recognition Of Net Actuarial Loss For Pension Settlement Costs, Net of Tax	25
Other Comprehensive Income (loss) and Settlement Costs, Net of Tax	61	6
Balance	$ (326)	$ (347)