Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ 5,614
Other comprehensive income (loss), net of tax	6	$ 3
Balance	5,254
Defined Benefit Plans [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(584)	(532)
Amounts reclassified from accumulated other comprehensive loss, before tax	11	7
Net current-period other comprehensive income (loss), before tax	11	7
Balance	(573)	(525)
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	(2)	2
Net current-period other comprehensive income (loss), before tax	(2)	2
Balance	18	(117)
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 before reclassifications, tax		(2)
Amounts reclassified from AOCI, tax	(3)	(4)
Net current-period other comprehensive income (loss), tax	(3)	(6)
Balance	208	241
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (353)	(404)
Other Comprehensive income (loss), before reclassifications, Net of Tax		(2)
Amounts reclassified from accumulated other comprehensive loss, net of tax	$ 6	5
Other comprehensive income (loss), net of tax	6	3
Balance	$ (347)	$ (401)