Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018
Deferred Tax
Balance	$ (140)	$ (48)		$ (136)		$ (39)
Changes in value	7	0		4		(6)
Amounts reclassified to earnings	(2)	(4)		(3)		(7)
Amounts reclassified to retained earnings						0
Balance	(135)	(52)		(135)		(52)
Net of Tax
Balance	9,802	8,388	[1]	10,042	[1],[2]	8,788	[1]
Changes in value	(20)	2		(13)		15
Amounts reclassified to earnings	6	10		9		23
Amounts reclassified to retained earnings						7
Balance	10,320	8,691	[1]	10,320		8,691	[1]
Pension and Other Postretirement Liabilities
Before Tax
Balance	(654)	(1,063)		(663)		(1,102)
Changes in value	(29)	1		(24)		24
Amounts reclassified to earnings	8	14		12		30
Amounts reclassified to retained earnings						0
Balance	(675)	(1,048)		(675)		(1,048)
Investments and Other
Before Tax
Balance	1	(3)		(4)		(6)
Changes in value	2	1		7		(3)
Amounts reclassified to earnings	0	0		0		0
Amounts reclassified to retained earnings						7
Balance	3	(2)		3		(2)
Accumulated Other Comprehensive Income (Loss)
Net of Tax
Balance	(793)	(1,114)	[1]	(803)	[1]	(1,147)	[1]
Balance	$ (807)	$ (1,102)	[1]	$ (807)		$ (1,102)	[1]

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.