Intangible Assets Including Goodwill - Goodwill by Segment (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Changes in Goodwill Balances
Beginning Balance	$ 36,265	$ 36,788
Goodwill Additions		34
Purchase Price Adjustments	0	(3)
Divestitures	(32)	(1)
Foreign Currency Translation and Other Adjustments	47	(553)
Ending Balance	36,281	36,265
Goodwill impairment losses	0
Goodwill accumulated impairment losses	0
Business Segments \| Cloud & Cognitive Software
Changes in Goodwill Balances
Beginning Balance	24,594	24,973
Goodwill Additions		9
Purchase Price Adjustments	0	0
Divestitures		(1)
Foreign Currency Translation and Other Adjustments	31	(388)
Ending Balance	24,625	24,594
Business Segments \| Global Business Services
Changes in Goodwill Balances
Beginning Balance	4,711	4,782
Goodwill Additions		24
Purchase Price Adjustments	0	(3)
Foreign Currency Translation and Other Adjustments	(3)	(92)
Ending Balance	4,708	4,711
Business Segments \| Global Technology Services
Changes in Goodwill Balances
Beginning Balance	3,988	4,044
Purchase Price Adjustments		0
Foreign Currency Translation and Other Adjustments	16	(56)
Ending Balance	4,003	3,988
Business Segments \| Systems
Changes in Goodwill Balances
Beginning Balance	1,847	1,862
Purchase Price Adjustments		0
Foreign Currency Translation and Other Adjustments	3	(15)
Ending Balance	1,851	1,847
Other \| Divested businesses
Changes in Goodwill Balances
Beginning Balance	1,126	1,127
Goodwill Additions		1
Purchase Price Adjustments		0
Divestitures	(32)	0
Foreign Currency Translation and Other Adjustments		(2)
Ending Balance	$ 1,094	$ 1,126

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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