Comprehensive Income (Loss) (Accumulated Other Comprehensive Income (Loss), Net Of Tax) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (4,900)	$ (4,394)
Net current-period other comprehensive income (loss)	11	86
Balance	(4,830)	(4,493)
Pension Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(699)	(684)
Other comprehensive income (loss) before reclassifications		1
Amounts reclassified from accumulated other comprehensive income (loss)	14	90
Net current-period other comprehensive income (loss)	14	91
Balance	(685)	(593)
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(56)	34
Other comprehensive income (loss) before reclassifications
Amounts reclassified from accumulated other comprehensive income (loss)	(3)	(5)
Net current-period other comprehensive income (loss)	(3)	(5)
Balance	(59)	29
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	230	176	$ 234	$ 204
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(755)	(650)
Other comprehensive income (loss) before reclassifications		1
Amounts reclassified from accumulated other comprehensive income (loss)	11	85
Net current-period other comprehensive income (loss)	11	86
Balance	$ (744)	$ (564)