Changes in Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 34,868	$ 34,573
Other comprehensive income	54	21
Ending balance	34,313	34,616
Gains (Losses) on Cash Flow Hedges
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(5)	(2)
OCI before reclassifications	0	(1)
Amounts reclassified from AOCI	0	0
Other comprehensive income	0	(1)
Ending balance	(5)	(3)
Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,372)	(3,165)
OCI before reclassifications	(2)	(7)
Amounts reclassified from AOCI	55	37
Other comprehensive income	53	30
Ending balance	(3,319)	(3,135)
Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(23)	(27)
OCI before reclassifications	1	(8)
Amounts reclassified from AOCI	0	0
Other comprehensive income	1	(8)
Ending balance	(22)	(35)
Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,400)	(3,194)
OCI before reclassifications	(1)	(16)
Amounts reclassified from AOCI	55	37
Other comprehensive income	54	21
Ending balance	$ (3,346)	$ (3,173)