Significant Accounting Policies - Basis of Presentation (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Net one-time benefit	$ 900
Measurement period adjustments recognized related to U.S. tax reform		$ 0	$ 100	$ 2,000
Other (income) and expense
Noncontrolling interest amounts, net of tax		$ 22	$ 25	$ 17
Cloud & Cognitive Software and Global Business Services
Segments impacted by realignment \| segment		2

The net benefit amount from the recognition of a deferred tax asset.

No definition available.

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- Definition

Represents the number of reportable segments that were impacted by the entity's offerings realignment.

+ References

No definition available.

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- Definition

Amount of Net Income (Loss) attributable to noncontrolling interest.

+ References

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- Definition

Amount of income tax expense (benefit) for measurement period adjustment from accounting for tax effect pursuant to Tax Cuts and Jobs Act.

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