Intangible Assets Including Goodwill - Goodwill by Segment (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Changes in Goodwill Balances
Beginning Balance	$ 36,265,000,000	$ 36,788,000,000
Goodwill Additions	23,102,000,000	34,000,000
Purchase Price Adjustments	1,000,000	(3,000,000)
Divestitures	(1,126,000,000)	(1,000,000)
Foreign Currency Translation and Other Adjustments	(291,000,000)	(553,000,000)
Ending Balance	57,951,000,000	36,265,000,000
Goodwill impairment losses	0	0
Goodwill accumulated impairment losses	0	0
Business Segments \| Cloud & Cognitive Software
Changes in Goodwill Balances
Beginning Balance	24,594,000,000	24,973,000,000
Goodwill Additions	18,399,000,000	9,000,000
Purchase Price Adjustments	0	0
Divestitures		(1,000,000)
Foreign Currency Translation and Other Adjustments	(174,000,000)	(388,000,000)
Ending Balance	42,819,000,000	24,594,000,000
Business Segments \| Global Business Services
Changes in Goodwill Balances
Beginning Balance	4,711,000,000	4,782,000,000
Goodwill Additions	1,059,000,000	24,000,000
Purchase Price Adjustments	1,000,000	(3,000,000)
Divestitures	(1,000,000)
Foreign Currency Translation and Other Adjustments	(76,000,000)	(92,000,000)
Ending Balance	5,694,000,000	4,711,000,000
Business Segments \| Global Technology Services
Changes in Goodwill Balances
Beginning Balance	3,988,000,000	4,044,000,000
Goodwill Additions	3,119,000,000
Purchase Price Adjustments		0
Foreign Currency Translation and Other Adjustments	(41,000,000)	(56,000,000)
Ending Balance	7,066,000,000	3,988,000,000
Business Segments \| Systems
Changes in Goodwill Balances
Beginning Balance	1,847,000,000	1,862,000,000
Goodwill Additions	525,000,000
Purchase Price Adjustments		0
Foreign Currency Translation and Other Adjustments	(1,000,000)	(15,000,000)
Ending Balance	2,372,000,000	1,847,000,000
Other \| Divested businesses
Changes in Goodwill Balances
Beginning Balance	1,126,000,000	1,127,000,000
Goodwill Additions		1,000,000
Purchase Price Adjustments		0
Divestitures	$ (1,126,000,000)	0
Foreign Currency Translation and Other Adjustments		(2,000,000)
Ending Balance		$ 1,126,000,000

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