Comprehensive Income (Loss) (Accumulated Other Comprehensive Loss, Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (3,771)	$ 1,535	$ 1,600	$ 2,401	$ 2,396	$ 2,274	$ 1,600	$ 2,274
Net current-period other comprehensive income (loss)	8	9	8	(34)	71	(1)	25	36
Balance	(4,103)	(3,771)	1,535	1,943	2,401	2,396	(4,103)	1,943
Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(489)			(345)	(489)	(345)
Other comprehensive income (loss) before reclassifications								1
Amounts reclassified from accumulated other comprehensive loss to net loss							31	38
Net current-period other comprehensive income (loss)							31	39
Balance	(541)			(306)			(541)	(306)
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			26			(21)	26	(21)
Other comprehensive income (loss) before reclassifications								1
Amounts reclassified from accumulated other comprehensive loss to net loss							(6)	(4)
Net current-period other comprehensive income (loss)							(6)	(3)
Balance	24			(24)			24	(24)
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	163			206			163	206	$ 250	$ 223
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(525)	(534)	(463)	(296)	(367)	(366)	(463)	(366)
Other comprehensive income (loss) before reclassifications								2
Amounts reclassified from accumulated other comprehensive loss to net loss							25	34
Net current-period other comprehensive income (loss)	8	9	8	(34)	71	(1)	25	36
Balance	$ (517)	$ (525)	(534)	$ (330)	$ (296)	$ (367)	(517)	$ (330)
Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effect adjustment, Net of tax							(79)
Accounting Standards Update 2018-02 [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effect adjustment, Net of tax							(83)
Accounting Standards Update 2018-02 [Member] | Postretirement Benefits Other Than Pensions (OPEB) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effect adjustment, Net of tax							4
Accounting Standards Update 2018-02 [Member] | Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effect adjustment, Net of tax			$ (79)				$ (79)