Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 3,658
Other comprehensive income, net of tax	$ 5	$ 2	13	$ 8
Balance	5,855		5,855
Defined Benefit Plans [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(532)	(411)
Amounts reclassified from accumulated other comprehensive loss, before tax			22	14
Net current-period other comprehensive income (loss), before tax			22	14
Balance	(510)	(397)	(510)	(397)
Defined Benefit Plans [Member] | Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(119)	(5)
Amounts reclassified from accumulated other comprehensive loss, before tax			3	(1)
Net current-period other comprehensive income (loss), before tax			3	(1)
Balance	(116)	(6)	(116)	(6)
Defined Benefit Plans [Member] | Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			247	156
Other comprehensive income before reclassifications, tax			(2)
Amounts reclassified from accumulated other comprehensive loss, tax			(10)	(5)
Net current-period other comprehensive income (loss), tax			(12)	(5)
Balance	235	151	235	151
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(404)	(260)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax			(2)
Amounts reclassified from accumulated other comprehensive loss, net of tax			15	8
Other comprehensive income, net of tax			13	8
Balance	$ (391)	$ (252)	$ (391)	$ (252)