Intangible Assets Including Goodwill - Goodwill by Segment (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Changes in Goodwill Balances
Beginning Balance	$ 36,788	$ 36,199
Goodwill Additions		16
Purchase Price Adjustments	0	(38)
Divestitures	(1)	(20)
Foreign Currency Translation and Other Adjustments	(56)	631
Ending Balance	36,732	36,788
Goodwill impairment losses	0	0
Goodwill accumulated impairment losses	0	0
Cognitive Solutions
Changes in Goodwill Balances
Beginning Balance	19,665	19,484
Goodwill Additions		3
Purchase Price Adjustments	0	(38)
Divestitures	(1)	(20)
Foreign Currency Translation and Other Adjustments	(76)	235
Ending Balance	19,589	19,665
Global Business Services
Changes in Goodwill Balances
Beginning Balance	4,813	4,607
Purchase Price Adjustments		2
Foreign Currency Translation and Other Adjustments	39	204
Ending Balance	4,852	4,813
Technology Services & Cloud Platforms
Changes in Goodwill Balances
Beginning Balance	10,447	10,258
Goodwill Additions		13
Purchase Price Adjustments	0	(2)
Foreign Currency Translation and Other Adjustments	(15)	179
Ending Balance	10,433	10,447
Systems
Changes in Goodwill Balances
Beginning Balance	1,862	1,850
Purchase Price Adjustments		0
Foreign Currency Translation and Other Adjustments	(5)	13
Ending Balance	$ 1,858	$ 1,862

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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