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Acquisitions & Divestitures - Purchase Price Allocation (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Acquisitions
Goodwill	$ 61,378	$ 59,617	$ 58,222
2021 Acquisitions \| Client relationships \| Minimum
Acquisitions
Weighted average useful life	4 years
2021 Acquisitions \| Client relationships \| Maximum
Acquisitions
Weighted average useful life	10 years
2021 Acquisitions \| Completed technology \| Minimum
Acquisitions
Weighted average useful life	4 years
2021 Acquisitions \| Completed technology \| Maximum
Acquisitions
Weighted average useful life	7 years
2021 Acquisitions \| Trademarks \| Minimum
Acquisitions
Weighted average useful life	1 year
2021 Acquisitions \| Trademarks \| Maximum
Acquisitions
Weighted average useful life	6 years
Turbonomic
Acquisitions
Current assets	$ 126
Goodwill	1,439
Total assets acquired	1,990
Current liabilities	49
Noncurrent liabilities	113
Total liabilities assumed	161
Total purchase price	$ 1,829
Weighted average useful life	8 years 10 months 24 days
Turbonomic \| Client relationships
Acquisitions
Intangible assets	$ 290
Turbonomic \| Completed technology
Acquisitions
Intangible assets	117
Turbonomic \| Trademarks
Acquisitions
Intangible assets	18
Other Acquisitions 2021
Acquisitions
Current assets	101
Property, plant, and equipment/noncurrent assets	6
Goodwill	912
Total assets acquired	1,356
Current liabilities	59
Noncurrent liabilities	76
Total liabilities assumed	135
Total purchase price	$ 1,220
Weighted average useful life	6 years 6 months
Other Acquisitions 2021 \| Client relationships
Acquisitions
Intangible assets	$ 173
Other Acquisitions 2021 \| Completed technology
Acquisitions
Intangible assets	134
Other Acquisitions 2021 \| Trademarks
Acquisitions
Intangible assets	29
Cloud & Cognitive Software \| Turbonomic
Acquisitions
Goodwill	1,372
Cloud & Cognitive Software \| Other Acquisitions 2021
Acquisitions
Goodwill	283
Global Business Services \| Turbonomic
Acquisitions
Goodwill	67
Global Business Services \| Other Acquisitions 2021
Acquisitions
Goodwill	$ 628