Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segments
Revenue	$ 18,756	$ 19,153	$ 57,830	$ 56,597
Pre-tax income from continuing operations	2,996	3,065	6,908	6,931
Business Segments, Excluding Intersegment Revenue
Segments
Revenue	18,694	19,098	57,655	56,405
Business Segments, Excluding Intersegment Revenue | Cognitive Solutions
Segments
Revenue	4,148	4,400	13,027	13,021
Business Segments, Excluding Intersegment Revenue | Global Business Services
Segments
Revenue	4,130	4,093	12,495	12,196
Business Segments, Excluding Intersegment Revenue | Technology Services & Cloud Platforms
Segments
Revenue	8,292	8,457	25,533	25,079
Business Segments, Excluding Intersegment Revenue | Systems
Segments
Revenue	1,736	1,721	5,412	4,863
Business Segments, Excluding Intersegment Revenue | Global Financing
Segments
Revenue	388	427	1,188	1,246
Internal transactions
Segments
Revenue	(1,476)	(1,384)	(4,738)	(4,265)
Pre-tax income from continuing operations	(116)	(126)	(625)	(515)
Internal transactions | Cognitive Solutions
Segments
Revenue	(639)	(629)	(2,122)	(2,001)
Internal transactions | Global Business Services
Segments
Revenue	(77)	(92)	(249)	(271)
Internal transactions | Technology Services & Cloud Platforms
Segments
Revenue	(240)	(164)	(550)	(497)
Internal transactions | Systems
Segments
Revenue	(181)	(227)	(576)	(571)
Internal transactions | Global Financing
Segments
Revenue	(338)	(272)	(1,240)	(925)
Business Segments
Segments
Revenue	20,170	20,482	62,392	60,670
Pre-tax income from continuing operations	$ 3,801	$ 3,843	$ 9,616	$ 9,459
Revenue year-to-year change (as a percent)	(1.50%)		2.80%
Pre-tax income year-to-year change (as a percent)	(1.10%)		1.70%
Pre-tax income/(loss) margin (as a percent)	18.80%	18.80%	15.40%	15.60%
Business Segments | Cognitive Solutions
Segments
Revenue	$ 4,787	$ 5,030	$ 15,149	$ 15,022
Pre-tax income from continuing operations	$ 1,629	$ 1,643	$ 4,718	$ 4,522
Revenue year-to-year change (as a percent)	(4.80%)		0.80%
Pre-tax income year-to-year change (as a percent)	(0.90%)		4.30%
Pre-tax income/(loss) margin (as a percent)	34.00%	32.70%	31.10%	30.10%
Business Segments | Global Business Services
Segments
Revenue	$ 4,207	$ 4,185	$ 12,744	$ 12,467
Pre-tax income from continuing operations	$ 579	$ 442	$ 1,109	$ 1,035
Revenue year-to-year change (as a percent)	0.50%		2.20%
Pre-tax income year-to-year change (as a percent)	30.90%		7.10%
Pre-tax income/(loss) margin (as a percent)	13.80%	10.60%	8.70%	8.30%
Business Segments | Technology Services & Cloud Platforms
Segments
Revenue	$ 8,533	$ 8,621	$ 26,083	$ 25,576
Pre-tax income from continuing operations	$ 1,075	$ 1,177	$ 2,395	$ 2,845
Revenue year-to-year change (as a percent)	(1.00%)		2.00%
Pre-tax income year-to-year change (as a percent)	(8.60%)		(15.80%)
Pre-tax income/(loss) margin (as a percent)	12.60%	13.70%	9.20%	11.10%
Business Segments | Systems
Segments
Revenue	$ 1,917	$ 1,948	$ 5,989	$ 5,434
Pre-tax income from continuing operations	$ 209	$ 337	$ 352	$ 222
Revenue year-to-year change (as a percent)	(1.60%)		10.20%
Pre-tax income year-to-year change (as a percent)	(37.90%)		58.80%
Pre-tax income/(loss) margin (as a percent)	10.90%	17.30%	5.90%	4.10%
Business Segments | Global Financing
Segments
Revenue	$ 726	$ 698	$ 2,428	$ 2,171
Pre-tax income from continuing operations	$ 308	$ 243	$ 1,042	$ 835
Revenue year-to-year change (as a percent)	4.00%		11.80%
Pre-tax income year-to-year change (as a percent)	26.90%		24.80%
Pre-tax income/(loss) margin (as a percent)	42.50%	34.80%	42.90%	38.50%