Intangible Assets Including Goodwill - Goodwill by Segment (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Changes in Goodwill Balances
Beginning Balance	$ 36,199	$ 32,021
Goodwill Additions	16	4,244
Purchase Price Adjustments	(36)	(7)
Divestitures	(15)	(18)
Foreign Currency Translation and Other Adjustments	618	(42)
Ending Balance	36,782	36,199
Goodwill impairment losses	0	0
Goodwill accumulated impairment losses	0	0
Cognitive Solutions
Changes in Goodwill Balances
Beginning Balance	19,484	15,621
Goodwill Additions	3	3,821
Purchase Price Adjustments	(36)	5
Divestitures	(15)	(12)
Foreign Currency Translation and Other Adjustments	243	48
Ending Balance	19,679	19,484
Global Business Services
Changes in Goodwill Balances
Beginning Balance	4,607	4,396
Goodwill Additions		303
Purchase Price Adjustments	4	4
Divestitures		(1)
Foreign Currency Translation and Other Adjustments	186	(95)
Ending Balance	4,797	4,607
Technology Services & Cloud Platforms
Changes in Goodwill Balances
Beginning Balance	10,258	10,156
Goodwill Additions	13	119
Purchase Price Adjustments	(3)	(12)
Divestitures		(5)
Foreign Currency Translation and Other Adjustments	175	(1)
Ending Balance	10,444	10,258
Systems
Changes in Goodwill Balances
Beginning Balance	1,850	1,848
Purchase Price Adjustments		(4)
Foreign Currency Translation and Other Adjustments	13	5
Ending Balance	$ 1,863	$ 1,850

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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