Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Information
Revenue (excluding other revenue not allocated to segments)	$ 18,083	$ 18,619
Revenue	18,155	18,684
Pre-tax income from continuing operations	1,424	1,034
Cognitive Solutions
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,062	3,979
Global Business Services
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,006	4,131
Technology Services & Cloud Platforms
Segment Information
Revenue (excluding other revenue not allocated to segments)	8,216	8,424
Systems
Segment Information
Revenue (excluding other revenue not allocated to segments)	1,395	1,675
Global Financing
Segment Information
Revenue (excluding other revenue not allocated to segments)	405	410
Business Segments
Segment Information
Revenue	19,576	20,264
Pre-tax income from continuing operations	$ 2,377	$ 1,837
Revenue year-to-year change (as a percent)	(3.40%)
Pre-tax income year-to-year change (as a percent)	29.40%
Pre-tax income margin (as a percent)	12.10%	9.10%
Business Segments | Cognitive Solutions
Segment Information
Revenue	$ 4,778	$ 4,647
Pre-tax income from continuing operations	$ 1,274	$ 1,013
Revenue year-to-year change (as a percent)	2.80%
Pre-tax income year-to-year change (as a percent)	25.70%
Pre-tax income margin (as a percent)	26.70%	21.80%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,092	$ 4,245
Pre-tax income from continuing operations	$ 291	$ 190
Revenue year-to-year change (as a percent)	(3.60%)
Pre-tax income year-to-year change (as a percent)	53.40%
Pre-tax income margin (as a percent)	7.10%	4.50%
Business Segments | Technology Services & Cloud Platforms
Segment Information
Revenue	$ 8,376	$ 8,589
Pre-tax income from continuing operations	$ 687	$ 258
Revenue year-to-year change (as a percent)	(2.50%)
Pre-tax income year-to-year change (as a percent)	166.10%
Pre-tax income margin (as a percent)	8.20%	3.00%
Business Segments | Systems
Segment Information
Revenue	$ 1,562	$ 1,888
Pre-tax income from continuing operations	$ (186)	$ (10)
Revenue year-to-year change (as a percent)	(17.30%)
Pre-tax income margin (as a percent)	(11.90%)	(0.50%)
Business Segments | Global Financing
Segment Information
Revenue	$ 768	$ 896
Pre-tax income from continuing operations	$ 311	$ 386
Revenue year-to-year change (as a percent)	(14.30%)
Pre-tax income year-to-year change (as a percent)	(19.50%)
Pre-tax income margin (as a percent)	40.50%	43.10%
Internal transactions
Segment Information
Revenue	$ (1,492)	$ (1,645)
Pre-tax income from continuing operations	(227)	(355)
Internal transactions | Cognitive Solutions
Segment Information
Revenue	(716)	(668)
Internal transactions | Global Business Services
Segment Information
Revenue	(86)	(113)
Internal transactions | Technology Services & Cloud Platforms
Segment Information
Revenue	(160)	(165)
Internal transactions | Systems
Segment Information
Revenue	(167)	(212)
Internal transactions | Global Financing
Segment Information
Revenue	$ (363)	$ (486)