Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment
Revenue	$ 19,072	$ 18,155
Pre-tax income/(loss) from continuing operations	1,136	1,424
Business Segments, Excluding Intersegment Revenue
Segment
Revenue	19,003	18,083
Business Segments, Excluding Intersegment Revenue | Cognitive Solutions
Segment
Revenue	4,299	4,062
Business Segments, Excluding Intersegment Revenue | Global Business Services
Segment
Revenue	4,174	4,006
Business Segments, Excluding Intersegment Revenue | Technology Services & Cloud Platforms
Segment
Revenue	8,625	8,216
Business Segments, Excluding Intersegment Revenue | Systems
Segment
Revenue	1,500	1,395
Business Segments, Excluding Intersegment Revenue | Global Financing
Segment
Revenue	405	405
Internal transactions
Segment
Revenue	(1,592)	(1,492)
Pre-tax income/(loss) from continuing operations	(218)	(227)
Internal transactions | Cognitive Solutions
Segment
Revenue	(780)	(716)
Internal transactions | Global Business Services
Segment
Revenue	(89)	(86)
Internal transactions | Technology Services & Cloud Platforms
Segment
Revenue	(141)	(160)
Internal transactions | Systems
Segment
Revenue	(153)	(167)
Internal transactions | Global Financing
Segment
Revenue	(429)	(363)
Business Segments
Segment
Revenue	20,595	19,576
Pre-tax income/(loss) from continuing operations	$ 2,088	$ 2,346
Revenue year-to-year change (as a percent)	5.20%
Pre-tax income year-to-year change (as a percent)	(11.00%)
Pre-tax income margin (as a percent)	10.10%	12.00%
Business Segments | Cognitive Solutions
Segment
Revenue	$ 5,079	$ 4,778
Pre-tax income/(loss) from continuing operations	$ 1,333	$ 1,268
Revenue year-to-year change (as a percent)	6.30%
Pre-tax income year-to-year change (as a percent)	5.10%
Pre-tax income margin (as a percent)	26.20%	26.50%
Business Segments | Global Business Services
Segment
Revenue	$ 4,263	$ 4,092
Pre-tax income/(loss) from continuing operations	$ 145	$ 281
Revenue year-to-year change (as a percent)	4.20%
Pre-tax income year-to-year change (as a percent)	(48.60%)
Pre-tax income margin (as a percent)	3.40%	6.90%
Business Segments | Technology Services & Cloud Platforms
Segment
Revenue	$ 8,766	$ 8,376
Pre-tax income/(loss) from continuing operations	$ 436	$ 673
Revenue year-to-year change (as a percent)	4.70%
Pre-tax income year-to-year change (as a percent)	(35.20%)
Pre-tax income margin (as a percent)	5.00%	8.00%
Business Segments | Systems
Segment
Revenue	$ 1,653	$ 1,562
Pre-tax income/(loss) from continuing operations	$ (203)	$ (188)
Revenue year-to-year change (as a percent)	5.80%
Pre-tax income year-to-year change (as a percent)	7.90%
Pre-tax income margin (as a percent)	(12.30%)	(12.00%)
Business Segments | Global Financing
Segment
Revenue	$ 834	$ 768
Pre-tax income/(loss) from continuing operations	$ 377	$ 311
Revenue year-to-year change (as a percent)	8.60%
Pre-tax income year-to-year change (as a percent)	21.30%
Pre-tax income margin (as a percent)	45.10%	40.40%