Changes in Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 34,573	$ 33,070
Other comprehensive income	21	(18)
Ending balance	34,616	33,704
Gains (Losses) on Cash Flow Hedges
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(2)	(2)
OCI before reclassifications	(1)	0
Amounts reclassified from AOCI	0	0
Other comprehensive income	(1)	0
Ending balance	(3)	(2)
Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,165)	(2,960)
OCI before reclassifications	(7)	(38)
Amounts reclassified from AOCI	37	20
Other comprehensive income	30	(18)
Ending balance	(3,135)	(2,978)
Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(27)	(33)
OCI before reclassifications	(8)	2
Amounts reclassified from AOCI	0	0
Other comprehensive income	(8)	2
Ending balance	(35)	(31)
AOCI of Investments in Unconsolidated Affiliates
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance		0
OCI before reclassifications		(1)
Amounts reclassified from AOCI		0
Other comprehensive income		(1)
Ending balance		(1)
Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,194)	(2,995)
OCI before reclassifications	(16)	(37)
Amounts reclassified from AOCI	37	20
Other comprehensive income	21	(17)
Ending balance	$ (3,173)	$ (3,012)