Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Information
Revenue	$ 17,730	$ 17,571
Pre-tax income/(loss) from continuing operations	905	(49)
External Revenue
Segment Information
Revenue	17,707	17,508
External Revenue | Cloud & Cognitive Software
Segment Information
Revenue	5,437	5,238
External Revenue | Global Business Services
Segment Information
Revenue	4,234	4,136
External Revenue | Global Technology Services
Segment Information
Revenue	6,370	6,467
External Revenue | Systems
Segment Information
Revenue	1,427	1,368
External Revenue | Global Financing
Segment Information
Revenue	240	299
Internal transactions
Segment Information
Revenue	(1,557)	(1,514)
Pre-tax income/(loss) from continuing operations	(73)	(55)
Internal transactions | Cloud & Cognitive Software
Segment Information
Revenue	(832)	(813)
Internal transactions | Global Business Services
Segment Information
Revenue	(55)	(46)
Internal transactions | Global Technology Services
Segment Information
Revenue	(313)	(294)
Internal transactions | Systems
Segment Information
Revenue	(189)	(148)
Internal transactions | Global Financing
Segment Information
Revenue	(168)	(212)
Business Segments
Segment Information
Revenue	19,264	19,023
Pre-tax income/(loss) from continuing operations	$ 2,122	$ 1,003
Revenue year-to-year change (as a percent)	1.30%
Pre-tax income/(loss) year-to-year change (as a percent)	111.60%
Pre-tax income/(loss) margin (as a percent)	11.00%	5.30%
Business Segments | Cloud & Cognitive Software
Segment Information
Revenue	$ 6,269	$ 6,052
Pre-tax income/(loss) from continuing operations	$ 1,428	$ 933
Revenue year-to-year change (as a percent)	3.60%
Pre-tax income/(loss) year-to-year change (as a percent)	53.00%
Pre-tax income/(loss) margin (as a percent)	22.80%	15.40%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,289	$ 4,183
Pre-tax income/(loss) from continuing operations	$ 390	$ 271
Revenue year-to-year change (as a percent)	2.50%
Pre-tax income/(loss) year-to-year change (as a percent)	44.10%
Pre-tax income/(loss) margin (as a percent)	9.10%	6.50%
Business Segments | Global Technology Services
Segment Information
Revenue	$ 6,683	$ 6,761
Pre-tax income/(loss) from continuing operations	$ 140	$ (178)
Revenue year-to-year change (as a percent)	(1.20%)
Pre-tax income/(loss) margin (as a percent)	2.10%	(2.60%)
Business Segments | Systems
Segment Information
Revenue	$ 1,616	$ 1,516
Pre-tax income/(loss) from continuing operations	$ (2)	$ (217)
Revenue year-to-year change (as a percent)	6.60%
Pre-tax income/(loss) margin (as a percent)	(0.10%)	(14.30%)
Business Segments | Global Financing
Segment Information
Revenue	$ 408	$ 511
Pre-tax income/(loss) from continuing operations	$ 166	$ 194
Revenue year-to-year change (as a percent)	(20.20%)
Pre-tax income/(loss) year-to-year change (as a percent)	(14.20%)
Pre-tax income/(loss) margin (as a percent)	40.80%	37.90%