Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Information
Revenue	$ 18,745	$ 18,123	$ 36,474	$ 35,694
Pre-tax income from continuing operations	1,552	1,571	2,457	1,522
External Revenue
Segment Information
Revenue	18,740	18,072	36,447	35,581
External Revenue | Cloud & Cognitive Software
Segment Information
Revenue	6,098	5,748	11,534	10,987
External Revenue | Global Business Services
Segment Information
Revenue	4,341	3,890	8,575	8,027
External Revenue | Global Technology Services
Segment Information
Revenue	6,342	6,316	12,712	12,783
External Revenue | Systems
Segment Information
Revenue	1,717	1,852	3,144	3,220
External Revenue | Global Financing
Segment Information
Revenue	242	265	482	564
Internal transactions
Segment Information
Revenue	(1,611)	(1,582)	(3,168)	(3,096)
Pre-tax income from continuing operations	(139)	(121)	(212)	(176)
Internal transactions | Cloud & Cognitive Software
Segment Information
Revenue	(726)	(743)	(1,558)	(1,556)
Internal transactions | Global Business Services
Segment Information
Revenue	(58)	(55)	(113)	(101)
Internal transactions | Global Technology Services
Segment Information
Revenue	(326)	(304)	(639)	(599)
Internal transactions | Systems
Segment Information
Revenue	(241)	(240)	(430)	(388)
Internal transactions | Global Financing
Segment Information
Revenue	(260)	(241)	(428)	(453)
Business Segments
Segment Information
Revenue	20,351	19,655	39,615	38,677
Pre-tax income from continuing operations	$ 2,894	$ 2,744	$ 5,015	$ 3,747
Revenue year-to-year change (as a percent)	3.50%		2.40%
Pre-tax income year-to-year change (as a percent)	5.50%		33.90%
Pre-tax income margin (as a percent)	14.20%	14.00%	12.70%	9.70%
Business Segments | Cloud & Cognitive Software
Segment Information
Revenue	$ 6,824	$ 6,491	$ 13,093	$ 12,543
Pre-tax income from continuing operations	$ 1,720	$ 1,708	$ 3,147	$ 2,641
Revenue year-to-year change (as a percent)	5.10%		4.40%
Pre-tax income year-to-year change (as a percent)	0.70%		19.20%
Pre-tax income margin (as a percent)	25.20%	26.30%	24.00%	21.10%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,399	$ 3,945	$ 8,688	$ 8,128
Pre-tax income from continuing operations	$ 371	$ 362	$ 761	$ 633
Revenue year-to-year change (as a percent)	11.50%		6.90%
Pre-tax income year-to-year change (as a percent)	2.50%		20.30%
Pre-tax income margin (as a percent)	8.40%	9.20%	8.80%	7.80%
Business Segments | Global Technology Services
Segment Information
Revenue	$ 6,668	$ 6,621	$ 13,351	$ 13,382
Pre-tax income from continuing operations	$ 381	$ 250	$ 520	$ 72
Revenue year-to-year change (as a percent)	0.70%		(0.20%)
Pre-tax income year-to-year change (as a percent)	52.30%		624.60%
Pre-tax income margin (as a percent)	5.70%	3.80%	3.90%	0.50%
Business Segments | Systems
Segment Information
Revenue	$ 1,958	$ 2,092	$ 3,574	$ 3,608
Pre-tax income from continuing operations	$ 176	$ 248	$ 174	$ 31
Revenue year-to-year change (as a percent)	(6.40%)		(1.00%)
Pre-tax income year-to-year change (as a percent)	(28.80%)		466.30%
Pre-tax income margin (as a percent)	9.00%	11.80%	4.90%	0.90%
Business Segments | Global Financing
Segment Information
Revenue	$ 502	$ 506	$ 910	$ 1,017
Pre-tax income from continuing operations	$ 246	$ 176	$ 412	$ 370
Revenue year-to-year change (as a percent)	(0.70%)		(10.50%)
Pre-tax income year-to-year change (as a percent)	39.30%		11.30%
Pre-tax income margin (as a percent)	48.90%	34.90%	45.30%	36.40%