Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Information
Revenue	$ 17,618	$ 17,560	$ 54,093	$ 53,253
Pre-tax income from continuing operations	1,319	1,827	3,776	3,348
External Revenue
Segment Information
Revenue	17,601	17,510	54,047	53,090
External Revenue | Cloud & Cognitive Software
Segment Information
Revenue	5,692	5,553	17,227	16,540
External Revenue | Global Business Services
Segment Information
Revenue	4,427	3,965	13,002	11,992
External Revenue | Global Technology Services
Segment Information
Revenue	6,154	6,462	18,866	19,245
External Revenue | Systems
Segment Information
Revenue	1,107	1,257	4,251	4,477
External Revenue | Global Financing
Segment Information
Revenue	220	273	702	837
Internal transactions
Segment Information
Revenue	(1,463)	(1,683)	(4,631)	(4,780)
Pre-tax income from continuing operations	(57)	(158)	(269)	(334)
Internal transactions | Cloud & Cognitive Software
Segment Information
Revenue	(764)	(875)	(2,322)	(2,431)
Internal transactions | Global Business Services
Segment Information
Revenue	(53)	(49)	(166)	(150)
Internal transactions | Global Technology Services
Segment Information
Revenue	(317)	(312)	(956)	(911)
Internal transactions | Systems
Segment Information
Revenue	(176)	(240)	(606)	(628)
Internal transactions | Global Financing
Segment Information
Revenue	(153)	(208)	(581)	(660)
Business Segments
Segment Information
Revenue	19,064	19,193	58,678	57,870
Pre-tax income from continuing operations	$ 2,644	$ 2,962	$ 7,659	$ 6,708
Revenue year-to-year change (as a percent)	(0.70%)		1.40%
Pre-tax income year-to-year change (as a percent)	(10.70%)		14.20%
Pre-tax income margin (as a percent)	13.90%	15.40%	13.10%	11.60%
Business Segments | Cloud & Cognitive Software
Segment Information
Revenue	$ 6,456	$ 6,428	$ 19,549	$ 18,971
Pre-tax income from continuing operations	$ 1,675	$ 1,834	$ 4,822	$ 4,475
Revenue year-to-year change (as a percent)	0.40%		3.00%
Pre-tax income year-to-year change (as a percent)	(8.70%)		7.80%
Pre-tax income margin (as a percent)	25.90%	28.50%	24.70%	23.60%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,480	$ 4,014	$ 13,168	$ 12,142
Pre-tax income from continuing operations	$ 587	$ 570	$ 1,349	$ 1,203
Revenue year-to-year change (as a percent)	11.60%		8.50%
Pre-tax income year-to-year change (as a percent)	3.00%		12.10%
Pre-tax income margin (as a percent)	13.10%	14.20%	10.20%	9.90%
Business Segments | Global Technology Services
Segment Information
Revenue	$ 6,471	$ 6,774	$ 19,822	$ 20,155
Pre-tax income from continuing operations	$ 383	$ 399	$ 903	$ 471
Revenue year-to-year change (as a percent)	(4.50%)		(1.70%)
Pre-tax income year-to-year change (as a percent)	(4.10%)		91.90%
Pre-tax income margin (as a percent)	5.90%	5.90%	4.60%	2.30%
Business Segments | Systems
Segment Information
Revenue	$ 1,283	$ 1,497	$ 4,857	$ 5,106
Pre-tax income from continuing operations	$ (207)	$ (37)	$ (33)	$ (7)
Revenue year-to-year change (as a percent)	(14.30%)		(4.90%)
Pre-tax income margin (as a percent)	(16.10%)	(2.50%)	(0.70%)	(0.10%)
Business Segments | Global Financing
Segment Information
Revenue	$ 373	$ 480	$ 1,283	$ 1,497
Pre-tax income from continuing operations	$ 206	$ 196	$ 618	$ 566
Revenue year-to-year change (as a percent)	(22.30%)		(14.30%)
Pre-tax income year-to-year change (as a percent)	5.10%		9.10%
Pre-tax income margin (as a percent)	55.10%	40.70%	48.10%	37.80%