Change in Accounting Principle (Effect on Consolidated Statement of Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME
Change in retirement plans’ funded status	$ (453)	$ (857)	[1]	$ (45)	[1]
Benefit for income tax, net	2	156	[1]	1	[1]
Other comprehensive income, net of tax	268	(418)	[1],[2]	(308)	[1],[2]
Comprehensive income	3,435	3,066	[1]	3,037	[1]
As Calculated Under Previous Method
CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME
Change in retirement plans’ funded status	(491)	(886)		(61)
Benefit for income tax, net	10	162		5
Other comprehensive income, net of tax	238	(441)		(320)
Comprehensive income	3,405	3,043		3,025
Effect of the Change in Accounting Principle | Amortization Of Actuarial Gains And Losses For U.S. Government Defined-benefit Pension Plan Participants
CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME
Change in retirement plans’ funded status	38	29		16
Benefit for income tax, net	(8)	(6)		(4)
Other comprehensive income, net of tax	30	23		12
Comprehensive income	$ 30	$ 23		$ 12

[1] *Prior-period information has been restated for the retrospective application of a change in accounting principle related to the amortization of actuarial gains and losses for our qualified U.S. government pension plans, which we adopted in the fourth quarter of 2020. For further discussion of this change in accounting principle, see Note T to the Consolidated Financial Statements.

[2] Prior-period information has been restated for the retrospective application of a change in accounting principle related to the amortization of actuarial gains and losses for our qualified U.S. government pension plans, which we adopted in the fourth quarter of 2020. For further discussion of this change in accounting principle, see Note T to the Consolidated Financial Statements.