Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ 5,614
Other comprehensive income (loss), net of tax	$ 5	$ 5	11	$ 8
Balance	5,062		5,062
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			21	13
Net current-period other comprehensive income (loss), before tax			21	13
Balance	(563)	(519)	(563)	(519)
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			(4)	3
Net current-period other comprehensive income (loss), before tax			(4)	3
Balance	16	(116)	16	(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				(2)
Amounts reclassified from AOCI, tax			(6)	(6)
Net current-period other comprehensive income (loss), tax			(6)	(8)
Balance	205	239	205	239
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			11	10
Other comprehensive income (loss), net of tax			11	8
Balance	$ (342)	$ (396)	$ (342)	$ (396)