Acquisitions/Divestitures - Red Hat Acquisition (Details) - USD ($)		3 Months Ended		9 Months Ended
	Jul. 09, 2019	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Acquisitions
Goodwill		$ 57,951,000,000	$ 57,951,000,000	$ 57,951,000,000	$ 36,265,000,000	$ 36,788,000,000
Red Hat, Inc.
Acquisitions
Acquired intangible asset, weighted average useful life				10 years 10 months 24 days
Goodwill	$ 23,102,000,000	23,100,000,000	23,100,000,000	$ 23,100,000,000
Expected percent of goodwill deductible for tax purpose				6.00%
Remaining compensation expense		230,000,000	230,000,000	$ 230,000,000
Settlement of pre-existing relationships	$ 60,000,000
Revenue since date of acquisition			371,000,000
Pre-tax income since date of acquisition			(981,000,000)
Red Hat, Inc. | Cost of sales
Acquisitions
Compensation expense			16,000,000
Red Hat, Inc. | SG&A expense
Acquisitions
Acquisition costs		98,000,000		171,000,000
Compensation expense			101,000,000
Red Hat, Inc. | R&D expense
Acquisitions
Compensation expense			68,000,000
Red Hat, Inc. | Maximum
Acquisitions
Remaining compensation expense recognition period	3 years
Red Hat, Inc. | Minimum
Acquisitions
Remaining compensation expense recognition period	6 months
Red Hat, Inc. | 364-Day Bridge Facility | SG&A expense
Acquisitions
Amortized costs		0		55,000,000
Cloud & Cognitive Software | Red Hat, Inc.
Acquisitions
Goodwill		18,400,000,000	18,400,000,000	18,400,000,000
Global Technology Services | Red Hat, Inc.
Acquisitions
Goodwill		3,100,000,000	3,100,000,000	3,100,000,000
Global Business Services | Red Hat, Inc.
Acquisitions
Goodwill		1,100,000,000	1,100,000,000	1,100,000,000
Systems | Red Hat, Inc.
Acquisitions
Goodwill		$ 500,000,000	$ 500,000,000	$ 500,000,000