Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income
Beginning balance	$ 13,545	$ 17,877
Other comprehensive loss before reclassifications	(55)	(5)
Amounts reclassified from accumulated other comprehensive loss		(3)
Other comprehensive loss, net of tax	(55)	(8)	$ (195)
Ending balance	12,905	13,545	17,877
Accumulated other comprehensive income
Increase (Decrease) in Accumulated Other Comprehensive Income
Beginning balance	(179)	(171)
Ending balance	(234)	(179)	(171)
Pension Plans
Increase (Decrease) in Accumulated Other Comprehensive Income
Beginning balance	2	5
Other comprehensive loss before reclassifications	(15)	0
Amounts reclassified from accumulated other comprehensive loss		(3)
Other comprehensive loss, net of tax	(15)	(3)
Ending balance	(13)	2	5
Foreign Currency Translation Adjustments and Other
Increase (Decrease) in Accumulated Other Comprehensive Income
Beginning balance	(181)	(176)
Other comprehensive loss before reclassifications	(40)	(5)
Amounts reclassified from accumulated other comprehensive loss		0
Other comprehensive loss, net of tax	(40)	(5)
Ending balance	$ (221)	$ (181)	$ (176)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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