Shareholders' Equity - (Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments								$ 366
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		$ (3,818)	[1]	$ (3,400)
Other comprehensive income, pretax		266		(574)		$ (309)
Benefit from (provision for) income tax, net		2		156		1
Other comprehensive (loss) income, net amount		268
Ending balance		(3,550)		(3,818)	[1]	(3,400)
(ASU) 2016-01 and (ASU) 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments	[2],[3]						$ 0
Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		2		(71)		(94)
Other comprehensive income, pretax		366		97		36
Benefit from (provision for) income tax, net		(96)		(24)		(9)
Other comprehensive (loss) income, net amount		270		73		27
Ending balance		272		2		(71)
Gains (Losses) on Cash Flow Hedges | (ASU) 2016-01 and (ASU) 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments	[3]						(4)
Unrealized Gains (Losses) On Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		0		0		19
Other comprehensive income, pretax		0		0		0
Benefit from (provision for) income tax, net		0		0		0
Other comprehensive (loss) income, net amount		0		0		0
Ending balance		0		0		0
Unrealized Gains (Losses) On Securities | (ASU) 2016-01 and (ASU) 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments	[3]						(19)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		288		102		402
Other comprehensive income, pretax		353		186		(300)
Benefit from (provision for) income tax, net		0		0		0
Other comprehensive (loss) income, net amount		353		186		(300)
Ending balance		641		288		102
Foreign Currency Translation Adjustments | (ASU) 2016-01 and (ASU) 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments	[3]						0
Changes In Retirement Plans' Funded Status
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(4,108)		(3,431)		(2,846)
Other comprehensive income, pretax		(453)		(857)		(45)
Benefit from (provision for) income tax, net		98		180		10
Other comprehensive (loss) income, net amount		(355)		(677)		(35)
Ending balance		(4,463)		(4,108)		(3,431)
Changes In Retirement Plans' Funded Status | (ASU) 2016-01 and (ASU) 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments	[3]						(550)
AOCL
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments								$ 301
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance						(2,519)
Other comprehensive (loss) income, net amount				$ (418)		$ (308)
Ending balance		$ (3,550)
AOCL | (ASU) 2016-01 and (ASU) 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effects adjustments	[2],[3]						$ (573)

[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.

[3] Reflects the cumulative effects of Accounting Standards Update (ASU) 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, and ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, which we adopted on January 1, 2018.