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, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Deferred Tax
Balance	$ (5)	$ (135)	$ (160)		$ (136)
Changes in value	2	(87)	173		(83)
Amounts reclassified to earnings	(74)	1	(90)		(2)
Balance	(77)	(221)	(77)		(221)
Net of Tax
Balance	8,517	10,320	11,531	[1]	10,042
Changes in value	(9)	307	(607)		294
Amounts reclassified to earnings	259	(4)	316		5
Balance	7,003	11,301	7,003		11,301
Pension and Other Postretirement Liabilities
Before Tax
Balance	(1,257)	(675)	(560)		(663)
Changes in value	(11)	394	(781)		370
Amounts reclassified to earnings	333	(4)	406		8
Balance	(935)	(285)	(935)		(285)
Investments and Other
Before Tax
Balance	3	3	2		(4)
Changes in value	0	0	1		7
Amounts reclassified to earnings	0	(1)	0		(1)
Balance	3	2	3		2
Accumulated Other Comprehensive Income (Loss)
Net of Tax
Balance	(1,259)	(807)	(718)		(803)
Balance	$ (1,009)	$ (504)	$ (1,009)		$ (504)

[1] Transition adjustment due to the adoption of Accounting Standards Update No. 2016-13, Financial Instruments—Credit Losses. See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.