Comprehensive Loss (Accumulated Other Comprehensive Income (Loss), Net Of Tax) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (4,394)	$ 1,600	$ 2,274
Net current-period other comprehensive income (loss)	(105)	(108)	(97)
Balance	(4,900)	(4,394)	1,600
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	234	204	250	$ 223
Defined Benefit Plans [Member] | Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(684)	(489)	(345)
Other comprehensive income (loss) before reclassifications	(320)	(201)	(191)
Amounts reclassified from accumulated other comprehensive income (loss)	305	89	47
Net current-period other comprehensive income (loss)	(15)	(112)	(144)
Balance	(699)	(684)	(489)
Defined Benefit Plans [Member] | Pension Benefits [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(83)
Balance		(83)
Defined Benefit Plans [Member] | Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	34	26	(21)
Other comprehensive income (loss) before reclassifications	(76)	17	51
Amounts reclassified from accumulated other comprehensive income (loss)	(14)	(13)	(4)
Net current-period other comprehensive income (loss)	(90)	4	47
Balance	(56)	34	26
Defined Benefit Plans [Member] | Postretirement Costs [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	4
Balance		4
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(650)	(463)	(366)
Other comprehensive income (loss) before reclassifications	(396)	(184)	(140)
Amounts reclassified from accumulated other comprehensive income (loss)	291	76	43
Net current-period other comprehensive income (loss)	(105)	(108)	(97)
Balance	(755)	(650)	$ (463)
Accumulated Other Comprehensive Loss [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (79)
Balance		$ (79)