Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Information
Revenue	$ 18,123	$ 19,161	$ 35,694	$ 37,342	$ 77,000
Pre-tax income from continuing operations	1,571	2,768	1,522	4,651
External Revenue
Segment Information
Revenue	18,072	18,701	35,581	36,432
External Revenue | Cloud & Cognitive Software
Segment Information
Revenue	5,748	5,563	10,987	10,530
External Revenue | Global Business Services
Segment Information
Revenue	3,890	4,197	8,027	8,353
External Revenue | Global Technology Services
Segment Information
Revenue	6,316	6,837	12,783	13,711
External Revenue | Systems
Segment Information
Revenue	1,852	1,753	3,220	3,081
External Revenue | Global Financing
Segment Information
Revenue	265	351	564	757
Internal transactions
Segment Information
Revenue	(1,582)	(1,430)	(3,096)	(3,097)
Pre-tax income from continuing operations	(121)	(60)	(176)	(149)
Internal transactions | Cloud & Cognitive Software
Segment Information
Revenue	(743)	(607)	(1,556)	(1,448)
Internal transactions | Global Business Services
Segment Information
Revenue	(55)	(69)	(101)	(143)
Internal transactions | Global Technology Services
Segment Information
Revenue	(304)	(302)	(599)	(591)
Internal transactions | Systems
Segment Information
Revenue	(240)	(171)	(388)	(334)
Internal transactions | Global Financing
Segment Information
Revenue	(241)	(281)	(453)	(581)
Business Segments
Segment Information
Revenue	19,655	20,131	38,677	39,529
Pre-tax income from continuing operations	$ 2,744	$ 2,832	$ 3,747	$ 5,276
Revenue year-to-year change (as a percent)	(2.40%)		(2.20%)
Pre-tax income year-to-year change (as a percent)	(3.10%)		(29.00%)
Pre-tax income/(loss) margin (as a percent)	14.00%	14.10%	9.70%	13.30%
Business Segments | Cloud & Cognitive Software
Segment Information
Revenue	$ 6,491	$ 6,170	$ 12,543	$ 11,978
Pre-tax income from continuing operations	$ 1,708	$ 2,007	$ 2,641	$ 3,792
Revenue year-to-year change (as a percent)	5.20%		4.70%
Pre-tax income year-to-year change (as a percent)	(14.90%)		(30.40%)
Pre-tax income/(loss) margin (as a percent)	26.30%	32.50%	21.10%	31.70%
Business Segments | Global Business Services
Segment Information
Revenue	$ 3,945	$ 4,266	$ 8,128	$ 8,495
Pre-tax income from continuing operations	$ 362	$ 290	$ 633	$ 587
Revenue year-to-year change (as a percent)	(7.50%)		(4.30%)
Pre-tax income year-to-year change (as a percent)	25.10%		7.80%
Pre-tax income/(loss) margin (as a percent)	9.20%	6.80%	7.80%	6.90%
Business Segments | Global Technology Services
Segment Information
Revenue	$ 6,621	$ 7,139	$ 13,382	$ 14,303
Pre-tax income from continuing operations	$ 250	$ 235	$ 72	$ 510
Revenue year-to-year change (as a percent)	(7.30%)		(6.40%)
Pre-tax income year-to-year change (as a percent)	6.40%		(85.90%)
Pre-tax income/(loss) margin (as a percent)	3.80%	3.30%	0.50%	3.60%
Business Segments | Systems
Segment Information
Revenue	$ 2,092	$ 1,924	$ 3,608	$ 3,415
Pre-tax income from continuing operations	$ 248	$ 61	$ 31	$ (141)
Revenue year-to-year change (as a percent)	8.70%		5.70%
Pre-tax income year-to-year change (as a percent)	304.80%
Pre-tax income/(loss) margin (as a percent)	11.80%	3.20%	0.90%	(4.10%)
Business Segments | Global Financing
Segment Information
Revenue	$ 506	$ 632	$ 1,017	$ 1,338
Pre-tax income from continuing operations	$ 176	$ 239	$ 370	$ 527
Revenue year-to-year change (as a percent)	(20.00%)		(24.00%)
Pre-tax income year-to-year change (as a percent)	(26.20%)		(29.80%)
Pre-tax income/(loss) margin (as a percent)	34.90%	37.80%	36.40%	39.40%
Adjustment
Segment Information
Revenue					$ 300