Changes in Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 34,500	$ 34,313	$ 34,868	$ 34,871	$ 34,616	$ 34,573	$ 34,868	$ 34,573
Other comprehensive income	66	57	54	28	41	21	177	90
Ending balance	34,259	34,500	34,313	35,103	34,871	34,616	34,259	35,103
Gains (Losses) on Cash Flow Hedges
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(5)		(5)	(3)		(2)	(5)	(2)
OCI before reclassifications	0			(1)			(1)	(2)
Amounts reclassified from AOCI	0			0			0	0
Other comprehensive income	0			(1)			(1)	(2)
Ending balance	(6)	(5)		(4)	(3)		(6)	(4)
Gains (Losses) on Cash Flow Hedges | Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Ending balance	(5)						(5)
Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,264)		(3,372)	(3,096)		(3,165)	(3,372)	(3,165)
OCI before reclassifications	14			(13)			15	(17)
Amounts reclassified from AOCI	55			39			163	112
Other comprehensive income	69			26			178	95
Ending balance	(3,194)	(3,264)		(3,070)	(3,096)		(3,194)	(3,070)
Pension and Non-Pension Postretirement Benefit Plan Items | Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Ending balance	(3,195)						(3,195)
Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(20)		(23)	(33)		(27)	(23)	(27)
OCI before reclassifications	(3)			3			0	(3)
Amounts reclassified from AOCI	0			0			0	0
Other comprehensive income	(3)			3			0	(3)
Ending balance	(23)	(20)		(30)	(33)		(23)	(30)
Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,289)	(3,346)	(3,400)	(3,132)	(3,173)	(3,194)	(3,400)	(3,194)
OCI before reclassifications	11			(11)			14	(22)
Amounts reclassified from AOCI	55			39			163	112
Other comprehensive income	66	57	54	28	41	21	177	90
Ending balance	$ (3,223)	$ (3,289)	$ (3,346)	$ (3,104)	$ (3,132)	$ (3,173)	$ (3,223)	$ (3,104)