Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Information
Revenue (excluding other revenue not allocated to segments)	$ 19,098	$ 19,145	$ 56,405	$ 57,926
Revenue	19,153	19,226	56,597	58,149
Pre-tax income from continuing operations	3,065	3,263	6,931	7,345
Cognitive Solutions
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,400	4,235	13,021	12,889
Global Business Services
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,093	4,191	12,196	12,578
Technology Services & Cloud Platforms
Segment Information
Revenue (excluding other revenue not allocated to segments)	8,457	8,748	25,079	26,029
Systems
Segment Information
Revenue (excluding other revenue not allocated to segments)	1,721	1,558	4,863	5,184
Global Financing
Segment Information
Revenue (excluding other revenue not allocated to segments)	427	412	1,246	1,245
Business Segments
Segment Information
Revenue	20,482	20,613	60,670	62,599
Pre-tax income from continuing operations	$ 3,876	$ 3,897	$ 9,555	$ 9,636
Revenue year-to-year change (as a percent)	(0.60%)		(3.10%)
Pre-tax income year-to-year change (as a percent)	(0.50%)		(0.80%)
Pre-tax income margin (as a percent)	18.90%	18.90%	15.70%	15.40%
Business Segments | Cognitive Solutions
Segment Information
Revenue	$ 5,030	$ 4,902	$ 15,022	$ 14,818
Pre-tax income from continuing operations	$ 1,649	$ 1,574	$ 4,539	$ 4,039
Revenue year-to-year change (as a percent)	2.60%		1.40%
Pre-tax income year-to-year change (as a percent)	4.80%		12.40%
Pre-tax income margin (as a percent)	32.80%	32.10%	30.20%	27.30%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,185	$ 4,284	$ 12,467	$ 12,888
Pre-tax income from continuing operations	$ 453	$ 544	$ 1,065	$ 1,210
Revenue year-to-year change (as a percent)	(2.30%)		(3.30%)
Pre-tax income year-to-year change (as a percent)	(16.80%)		(12.00%)
Pre-tax income margin (as a percent)	10.80%	12.70%	8.50%	9.40%
Business Segments | Technology Services & Cloud Platforms
Segment Information
Revenue	$ 8,621	$ 8,929	$ 25,576	$ 26,530
Pre-tax income from continuing operations	$ 1,192	$ 1,288	$ 2,888	$ 2,825
Revenue year-to-year change (as a percent)	(3.40%)		(3.60%)
Pre-tax income year-to-year change (as a percent)	(7.50%)		2.20%
Pre-tax income margin (as a percent)	13.80%	14.40%	11.30%	10.60%
Business Segments | Systems
Segment Information
Revenue	$ 1,948	$ 1,734	$ 5,434	$ 5,778
Pre-tax income from continuing operations	$ 339	$ 136	$ 227	$ 354
Revenue year-to-year change (as a percent)	12.30%		(6.00%)
Pre-tax income year-to-year change (as a percent)	149.70%		(35.90%)
Pre-tax income margin (as a percent)	17.40%	7.80%	4.20%	6.10%
Business Segments | Global Financing
Segment Information
Revenue	$ 698	$ 763	$ 2,171	$ 2,585
Pre-tax income from continuing operations	$ 244	$ 355	$ 836	$ 1,208
Revenue year-to-year change (as a percent)	(8.50%)		(16.00%)
Pre-tax income year-to-year change (as a percent)	(31.40%)		(30.70%)
Pre-tax income margin (as a percent)	34.90%	46.50%	38.50%	46.70%
Internal transactions
Segment Information
Revenue	$ (1,384)	$ (1,468)	$ (4,265)	$ (4,673)
Pre-tax income from continuing operations	(126)	(185)	(515)	(873)
Internal transactions | Cognitive Solutions
Segment Information
Revenue	(629)	(667)	(2,001)	(1,929)
Internal transactions | Global Business Services
Segment Information
Revenue	(92)	(93)	(271)	(310)
Internal transactions | Technology Services & Cloud Platforms
Segment Information
Revenue	(164)	(180)	(497)	(501)
Internal transactions | Systems
Segment Information
Revenue	(227)	(176)	(571)	(594)
Internal transactions | Global Financing
Segment Information
Revenue	$ (272)	$ (352)	$ (925)	$ (1,340)