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, 2020	Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019
Deferred Tax
Balance	$ (144)	$ (140)	$ (160)		$ (136)
Changes in value	156	7	171		4
Amounts reclassified to earnings	(17)	(2)	(16)		(3)
Balance	(5)	(135)	(5)		(135)
Net of Tax
Balance	9,418	9,802	11,531	[1]	10,042
Changes in value	(548)	(20)	(598)		(13)
Amounts reclassified to earnings	60	6	57		9
Balance	8,517	10,320	8,517		10,320
Pension and Other Postretirement Liabilities
Before Tax
Balance	(613)	(654)	(560)		(663)
Changes in value	(721)	(29)	(770)		(24)
Amounts reclassified to earnings	77	8	73		12
Balance	(1,257)	(675)	(1,257)		(675)
Investments and Other
Before Tax
Balance	(14)	1	2		(4)
Changes in value	17	2	1		7
Amounts reclassified to earnings	0	0	0		0
Balance	3	3	3		3
Accumulated Other Comprehensive Income (Loss)
Net of Tax
Balance	(771)	(793)	(718)		(803)
Balance	$ (1,259)	$ (807)	$ (1,259)		$ (807)

[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.