Changes in Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 34,313	$ 34,868	$ 34,616	$ 34,573	$ 34,868	$ 34,573
Other comprehensive income	57	54	41	21	111	62
Ending balance	34,500	34,313	34,871	34,616	34,500	34,871
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		0		0	(1)
Amounts reclassified from AOCI	0		0		0	0
Other comprehensive income	0		0		0	(1)
Ending balance	(5)	(5)	(3)	(3)	(5)	(3)
Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,319)	(3,372)	(3,135)	(3,165)	(3,372)	(3,165)
OCI before reclassifications	0		2		(2)	(5)
Amounts reclassified from AOCI	55		37		110	74
Other comprehensive income	55		39		108	69
Ending balance	(3,264)	(3,319)	(3,096)	(3,135)	(3,264)	(3,096)
Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(22)	(23)	(35)	(27)	(23)	(27)
OCI before reclassifications	2		2		3	(6)
Amounts reclassified from AOCI	0		0		0	0
Other comprehensive income	2		2		3	(6)
Ending balance	(20)	(22)	(33)	(35)	(20)	(33)
Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,346)	(3,400)	(3,173)	(3,194)	(3,400)	(3,194)
OCI before reclassifications	2		4		1	(12)
Amounts reclassified from AOCI	55		37		110	74
Other comprehensive income	57	54	41	21	111	62
Ending balance	$ (3,289)	$ (3,346)	$ (3,132)	$ (3,173)	$ (3,289)	$ (3,132)