Comprehensive Income (Loss) (Accumulated Other Comprehensive Income (Loss), Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (4,394)	$ 1,600
Net current-period other comprehensive income (loss)	86	8
Balance	(4,493)	1,535
Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(684)	(489)
Other comprehensive income (loss) before reclassifications	1
Amounts reclassified from accumulated other comprehensive loss to net loss	90	10
Net current-period other comprehensive income (loss)	91	10
Impact of adoption of new accounting principle		(83)
Balance	(593)	(562)
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	34	26
Other comprehensive income (loss) before reclassifications
Amounts reclassified from accumulated other comprehensive loss to net loss	(5)	(2)
Net current-period other comprehensive income (loss)	(5)	(2)
Impact of adoption of new accounting principle		4
Balance	29	28
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	176	169	$ 204	$ 250
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(650)	(463)
Other comprehensive income (loss) before reclassifications	1
Amounts reclassified from accumulated other comprehensive loss to net loss	85	8
Net current-period other comprehensive income (loss)	86	8
Impact of adoption of new accounting principle		(79)
Balance	$ (564)	$ (534)