Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segments
Revenue	$ 18,028	$ 18,756	$ 55,370	$ 57,830
Pre-tax income/(loss) from continuing operations	1,522	2,996	6,173	6,908
Business Segments, Excluding Intersegment Revenue
Segments
Revenue	17,921	18,262	54,426	56,319
Business Segments, Excluding Intersegment Revenue | Cloud & Cognitive Software
Segments
Revenue	5,280	4,962	15,962	15,548
Business Segments, Excluding Intersegment Revenue | Global Business Services
Segments
Revenue	4,117	4,076	12,391	12,326
Business Segments, Excluding Intersegment Revenue | Global Technology Services
Segments
Revenue	6,700	7,101	20,412	21,846
Business Segments, Excluding Intersegment Revenue | Systems
Segments
Revenue	1,481	1,736	4,562	5,412
Business Segments, Excluding Intersegment Revenue | Global Financing
Segments
Revenue	343	388	1,100	1,188
Internal transactions
Segments
Revenue	(1,541)	(1,652)	(4,638)	(5,173)
Pre-tax income/(loss) from continuing operations	(84)	(118)	(233)	(615)
Internal transactions | Cloud & Cognitive Software
Segments
Revenue	(686)	(777)	(2,135)	(2,518)
Internal transactions | Global Business Services
Segments
Revenue	(70)	(77)	(213)	(249)
Internal transactions | Global Technology Services
Segments
Revenue	(287)	(279)	(879)	(589)
Internal transactions | Systems
Segments
Revenue	(195)	(181)	(528)	(576)
Internal transactions | Global Financing
Segments
Revenue	(302)	(338)	(884)	(1,240)
Business Segments
Segments
Revenue	19,462	19,914	59,065	61,492
Pre-tax income/(loss) from continuing operations	$ 2,661	$ 3,741	$ 7,941	$ 9,342
Revenue year-to-year change (as a percent)	(2.30%)		(3.90%)
Pre-tax income year-to-year change (as a percent)	(28.90%)		(15.00%)
Pre-tax income/(loss) margin (as a percent)	13.70%	18.80%	13.40%	15.20%
Business Segments | Cloud & Cognitive Software
Segments
Revenue	$ 5,966	$ 5,738	$ 18,097	$ 18,066
Pre-tax income/(loss) from continuing operations	$ 1,283	$ 2,050	$ 5,052	$ 5,760
Revenue year-to-year change (as a percent)	4.00%		0.20%
Pre-tax income year-to-year change (as a percent)	(37.40%)		(12.30%)
Pre-tax income/(loss) margin (as a percent)	21.50%	35.70%	27.90%	31.90%
Business Segments | Global Business Services
Segments
Revenue	$ 4,187	$ 4,153	$ 12,604	$ 12,575
Pre-tax income/(loss) from continuing operations	$ 573	$ 566	$ 1,188	$ 1,063
Revenue year-to-year change (as a percent)	0.80%		0.20%
Pre-tax income year-to-year change (as a percent)	1.20%		11.70%
Pre-tax income/(loss) margin (as a percent)	13.70%	13.60%	9.40%	8.50%
Business Segments | Global Technology Services
Segments
Revenue	$ 6,988	$ 7,380	$ 21,291	$ 22,435
Pre-tax income/(loss) from continuing operations	$ 490	$ 607	$ 1,000	$ 1,124
Revenue year-to-year change (as a percent)	(5.30%)		(5.10%)
Pre-tax income year-to-year change (as a percent)	(19.20%)		(11.00%)
Pre-tax income/(loss) margin (as a percent)	7.00%	8.20%	4.70%	5.00%
Business Segments | Systems
Segments
Revenue	$ 1,676	$ 1,917	$ 5,091	$ 5,989
Pre-tax income/(loss) from continuing operations	$ 39	$ 209	$ (101)	$ 352
Revenue year-to-year change (as a percent)	(12.60%)		(15.00%)
Pre-tax income year-to-year change (as a percent)	(81.30%)		(128.80%)
Pre-tax income/(loss) margin (as a percent)	2.30%	10.90%	(2.00%)	5.90%
Business Segments | Global Financing
Segments
Revenue	$ 645	$ 726	$ 1,983	$ 2,428
Pre-tax income/(loss) from continuing operations	$ 275	$ 308	$ 803	$ 1,042
Revenue year-to-year change (as a percent)	(11.20%)		(18.30%)
Pre-tax income year-to-year change (as a percent)	(10.80%)		(23.00%)
Pre-tax income/(loss) margin (as a percent)	42.60%	42.50%	40.50%	42.90%