Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 5,614
Other comprehensive income before reclassifications, before tax			105
Other comprehensive income (loss), net of tax	$ (61)	$ 5	(50)	$ 13
Balance	4,757		4,757
Defined Benefit Plans [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(584)	(532)
Other comprehensive income before reclassifications, before tax			(105)
Amounts reclassified from accumulated other comprehensive loss, before tax			30	22
Net current-period other comprehensive income (loss), before tax			(75)	22
Balance	(659)	(510)	(659)	(510)
Defined Benefit Plans [Member] | Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			20	(119)
Amounts reclassified from accumulated other comprehensive loss, before tax			(6)	3
Net current-period other comprehensive income (loss), before tax			(6)	3
Balance	14	(116)	14	(116)
Defined Benefit Plans [Member] | Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			211	247
Other comprehensive income (loss) before reclassifications, tax			40	(2)
Amounts reclassified from AOCI, tax			(9)	(10)
Net current-period other comprehensive income (loss), tax			31	(12)
Balance	242	235	242	235
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(353)	(404)
Other Comprehensive income (loss), before reclassifications, Net of Tax			(65)	(2)
Amounts reclassified from accumulated other comprehensive loss, net of tax			15	15
Other comprehensive income (loss), net of tax			(50)	13
Balance	$ (403)	$ (391)	$ (403)	$ (391)