Comprehensive Income (Loss) (Accumulated Other Comprehensive Income (Loss), Net of Tax) (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	$ (5,097)	$ (4,493)	$ (4,394)	$ (3,771)	$ 1,535		$ (4,394)
Other comprehensive income (loss) before reclassifications							376
Net current-period other comprehensive income (loss)	155	(423)	86	8	9	$ 8	(182)	$ 25
Impact of adoption of ASU 2018-02	(8,925)						(8,925)		$ (8,573)
Balance	(4,926)	(5,097)	(4,493)	(4,103)	(3,771)	1,535	(4,926)	(4,103)
Previously Reported [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance						1,600		1,600
Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			(684)			(489)	(684)	(489)
Other comprehensive income (loss) before reclassifications							(389)
Amounts reclassified from accumulated other comprehensive loss to net loss							233	31
Net current-period other comprehensive income (loss)							(156)	31
Impact of adoption of ASU 2018-02				(83)				(83)
Balance	(840)			(541)			(840)	(541)
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			34			26	34	26
Other comprehensive income (loss) before reclassifications							(16)
Amounts reclassified from accumulated other comprehensive loss to net loss							(10)	(6)
Net current-period other comprehensive income (loss)							(26)	(6)
Impact of adoption of ASU 2018-02				4				4
Balance	8			24			8	24
Deferred Taxes On Pension And OPEB Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred tax items	255			163			255	163	$ 204	$ 250
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	(987)	(564)	(650)	(525)	(534)	(79)	(650)	(79)
Other comprehensive income (loss) before reclassifications							(405)
Amounts reclassified from accumulated other comprehensive loss to net loss							223	25
Net current-period other comprehensive income (loss)	155	(423)	86	8	9	8	(182)	25
Impact of adoption of ASU 2018-02				(79)				(79)
Balance	$ (832)	$ (987)	(564)	$ (517)	$ (525)	(534)	(832)	(517)
Accumulated Other Comprehensive Loss [Member] | Previously Reported [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance			$ (650)			$ (463)	$ (650)	$ (463)