Changes in Accumulated Other Comprehensive Income - Schedule of Changes in AOCI (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
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	$ 34,871	$ 34,616	$ 34,573	$ 33,907	$ 33,704	$ 33,070	$ 34,573	$ 33,070
Other comprehensive income	28			30			90	33
Ending balance	35,103	34,871	34,616	34,352	33,907	33,704	35,103	34,352
Gains (Losses) on Cash Flow Hedges
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3)		(2)	(2)		(2)	(2)	(2)
OCI before reclassifications	(1)			0			(2)	0
Amounts reclassified from AOCI	0			0			0	0
Other comprehensive income	(1)			0			(2)	0
Ending balance	(4)	(3)		(2)	(2)		(4)	(2)
Pension and Non-Pension Postretirement Benefit Plan Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,096)		(3,165)	(2,957)		(2,960)	(3,165)	(2,960)
OCI before reclassifications	(13)			6			(17)	(32)
Amounts reclassified from AOCI	39			21			112	62
Other comprehensive income	26			27			95	30
Ending balance	(3,070)	(3,096)		(2,930)	(2,957)		(3,070)	(2,930)
Foreign Currency Items
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(33)		(27)	(29)		(33)	(27)	(33)
OCI before reclassifications	3			(2)			(3)	2
Amounts reclassified from AOCI	0			0			0	0
Other comprehensive income	3			(2)			(3)	2
Ending balance	(30)	(33)		(31)	(29)		(30)	(31)
AOCI of Investments in Unconsolidated Affiliates
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance				(2)		0		0
OCI before reclassifications				0				(2)
Amounts reclassified from AOCI				2				2
Other comprehensive income				2				0
Ending balance				0	(2)			0
Total
Movement in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(3,132)	(3,173)	(3,194)	(2,990)	(3,012)	(2,995)	(3,194)	(2,995)
OCI before reclassifications	(11)			4			(22)	(32)
Amounts reclassified from AOCI	39			23			112	64
Other comprehensive income	28	41	21	27	22	(17)	90	32
Ending balance	$ (3,104)	$ (3,132)	$ (3,173)	$ (2,963)	$ (2,990)	$ (3,012)	$ (3,104)	$ (2,963)