Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Information
Revenue (excluding other revenue not allocated to segments)	$ 19,224	$ 20,162	$ 37,307	$ 38,781
Revenue	19,289	20,238	37,443	38,923
Pre-tax income from continuing operations	2,443	3,049	3,867	4,082
Cognitive Solutions
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,559	4,675	8,621	8,654
Global Business Services
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,097	4,255	8,103	8,387
Technology Services & Cloud Platforms
Segment Information
Revenue (excluding other revenue not allocated to segments)	8,406	8,857	16,622	17,280
Systems
Segment Information
Revenue (excluding other revenue not allocated to segments)	1,747	1,950	3,142	3,626
Global Financing
Segment Information
Revenue (excluding other revenue not allocated to segments)	415	424	819	834
Business Segments
Segment Information
Revenue	20,612	21,723	40,188	41,987
Pre-tax income from continuing operations	$ 3,302	$ 3,901	$ 5,679	$ 5,739
Revenue year-to-year change (as a percent)	(5.10%)		(4.30%)
Pre-tax income year-to-year change (as a percent)	(15.40%)		(1.00%)
Pre-tax income margin (as a percent)	16.00%	18.00%	14.10%	13.70%
Business Segments | Cognitive Solutions
Segment Information
Revenue	$ 5,214	$ 5,269	$ 9,992	$ 9,916
Pre-tax income from continuing operations	$ 1,615	$ 1,451	$ 2,889	$ 2,465
Revenue year-to-year change (as a percent)	(1.00%)		0.80%
Pre-tax income year-to-year change (as a percent)	11.30%		17.20%
Pre-tax income margin (as a percent)	31.00%	27.50%	28.90%	24.90%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,190	$ 4,359	$ 8,282	$ 8,603
Pre-tax income from continuing operations	$ 321	$ 476	$ 612	$ 665
Revenue year-to-year change (as a percent)	(3.90%)		(3.70%)
Pre-tax income year-to-year change (as a percent)	(32.50%)		(8.00%)
Pre-tax income margin (as a percent)	7.70%	10.90%	7.40%	7.70%
Business Segments | Technology Services & Cloud Platforms
Segment Information
Revenue	$ 8,579	$ 9,013	$ 16,955	$ 17,602
Pre-tax income from continuing operations	$ 1,009	$ 1,279	$ 1,696	$ 1,537
Revenue year-to-year change (as a percent)	(4.80%)		(3.70%)
Pre-tax income year-to-year change (as a percent)	(21.10%)		10.30%
Pre-tax income margin (as a percent)	11.80%	14.20%	10.00%	8.70%
Business Segments | Systems
Segment Information
Revenue	$ 1,924	$ 2,156	$ 3,486	$ 4,044
Pre-tax income from continuing operations	$ 74	$ 229	$ (112)	$ 218
Revenue year-to-year change (as a percent)	(10.80%)		(13.80%)
Pre-tax income year-to-year change (as a percent)	(67.50%)
Pre-tax income margin (as a percent)	3.90%	10.60%	(3.20%)	5.40%
Business Segments | Global Financing
Segment Information
Revenue	$ 705	$ 926	$ 1,473	$ 1,822
Pre-tax income from continuing operations	$ 282	$ 467	$ 593	$ 853
Revenue year-to-year change (as a percent)	(23.90%)		(19.10%)
Pre-tax income year-to-year change (as a percent)	(39.60%)		(30.50%)
Pre-tax income margin (as a percent)	40.00%	50.50%	40.30%	46.80%
Internal transactions
Segment Information
Revenue	$ (1,388)	$ (1,561)	$ (2,881)	$ (3,206)
Pre-tax income from continuing operations	(161)	(334)	(388)	(689)
Internal transactions | Cognitive Solutions
Segment Information
Revenue	(655)	(594)	(1,371)	(1,262)
Internal transactions | Global Business Services
Segment Information
Revenue	(93)	(103)	(179)	(216)
Internal transactions | Technology Services & Cloud Platforms
Segment Information
Revenue	(173)	(156)	(333)	(321)
Internal transactions | Systems
Segment Information
Revenue	(177)	(206)	(344)	(418)
Internal transactions | Global Financing
Segment Information
Revenue	$ (290)	$ (502)	$ (653)	$ (988)