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Acquisitions & Divestitures - Purchase Price Allocation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Acquisitions
Goodwill	$ 59,984	$ 59,617	$ 58,222
2021 Acquisitions
Acquisitions
Current assets	72
Property, plant, and equipment/noncurrent assets	4
Goodwill	746
Total assets acquired	1,097
Current liabilities	48
Noncurrent liabilities	62
Total liabilities assumed	110
Total purchase price	$ 987
Acquired intangible asset, weighted average useful life	6 years 9 months 18 days
2021 Acquisitions \| Client relationships
Acquisitions
Intangible assets	$ 134
Acquired intangible asset, weighted average useful life	7 years
2021 Acquisitions \| Completed technology
Acquisitions
Intangible assets	$ 114
2021 Acquisitions \| Completed technology \| Minimum
Acquisitions
Acquired intangible asset, weighted average useful life	3 years
2021 Acquisitions \| Completed technology \| Maximum
Acquisitions
Acquired intangible asset, weighted average useful life	7 years
2021 Acquisitions \| Trademarks
Acquisitions
Intangible assets	$ 27
2021 Acquisitions \| Trademarks \| Minimum
Acquisitions
Acquired intangible asset, weighted average useful life	2 years
2021 Acquisitions \| Trademarks \| Maximum
Acquisitions
Acquired intangible asset, weighted average useful life	6 years
Cloud & Cognitive Software \| 2021 Acquisitions
Acquisitions
Goodwill	$ 245
Global Business Services \| 2021 Acquisitions
Acquisitions
Goodwill	$ 501