Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income, Net of Tax (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018		Sep. 30, 2019		Sep. 30, 2018
Deferred Tax
Balance	$ (135)	$ (52)		$ (136)		$ (39)
Changes in value	(87)	0		(83)		(6)
Amounts reclassified to earnings	1	(3)		(2)		(10)
Amounts reclassified to retained earnings						0
Balance	(221)	(55)		(221)		(55)
Net of Tax
Balance	10,320	8,691	[1]	10,042	[1],[2]	8,788	[1]
Changes in value	307	1		294		16
Amounts reclassified to earnings	(4)	12		5		35
Amounts reclassified to retained earnings						7
Balance	11,301	9,518	[1]	11,301		9,518	[1]
Pension and Other Postretirement Liabilities
Before Tax
Balance	(675)	(1,048)		(663)		(1,102)
Changes in value	394	0		370		24
Amounts reclassified to earnings	(4)	15		8		45
Amounts reclassified to retained earnings						0
Balance	(285)	(1,033)		(285)		(1,033)
Investments and Other
Before Tax
Balance	3	(2)		(4)		(6)
Changes in value	0	1		7		(2)
Amounts reclassified to earnings	(1)	0		(1)		0
Amounts reclassified to retained earnings						7
Balance	2	(1)		2		(1)
Accumulated Other Comprehensive Income (Loss)
Net of Tax
Balance	(807)	(1,102)	[1]	(803)	[1]	(1,147)	[1]
Balance	$ (504)	$ (1,089)	[1]	$ (504)		$ (1,089)	[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.