Segments - Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)
Segment Information
Number of business segments (in segments) | segment			5
Revenue (excluding other revenue not allocated to segments)	$ 20,162	$ 20,778	$ 38,781	$ 40,301
Revenue	20,238	20,813	38,923	40,403
Pre-tax income/(loss) from continuing operations	3,049	4,224	4,082	7,225
Cognitive Solutions
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,675	4,516	8,654	8,564
Global Business Services
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,255	4,345	8,387	8,663
Technology Services & Cloud Platforms
Segment Information
Revenue (excluding other revenue not allocated to segments)	8,857	8,898	17,280	17,452
Systems
Segment Information
Revenue (excluding other revenue not allocated to segments)	1,950	2,541	3,626	4,683
Global Financing
Segment Information
Revenue (excluding other revenue not allocated to segments)	424	478	834	939
Business Segments
Segment Information
Revenue	21,723	22,506	41,987	43,719
Pre-tax income/(loss) from continuing operations	$ 3,901	$ 5,033	$ 5,739	$ 9,056
Revenue year-to-year change (as a percent)	(3.50%)		(4.00%)
Pre-tax income year-to-year change (as a percent)	(22.50%)		(36.60%)
Pre-tax income margin (as a percent)	18.00%	22.40%	13.70%	20.70%
Business Segments | Cognitive Solutions
Segment Information
Revenue	$ 5,269	$ 5,049	$ 9,916	$ 9,731
Pre-tax income/(loss) from continuing operations	$ 1,451	$ 1,825	$ 2,465	$ 3,353
Revenue year-to-year change (as a percent)	4.40%		1.90%
Pre-tax income year-to-year change (as a percent)	(20.50%)		(26.50%)
Pre-tax income margin (as a percent)	27.50%	36.10%	24.90%	34.50%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,359	$ 4,475	$ 8,603	$ 8,923
Pre-tax income/(loss) from continuing operations	$ 476	$ 643	$ 665	$ 1,231
Revenue year-to-year change (as a percent)	(2.60%)		(3.60%)
Pre-tax income year-to-year change (as a percent)	(26.10%)		(46.00%)
Pre-tax income margin (as a percent)	10.90%	14.40%	7.70%	13.80%
Business Segments | Technology Services & Cloud Platforms
Segment Information
Revenue	$ 9,013	$ 9,071	$ 17,602	$ 17,791
Pre-tax income/(loss) from continuing operations	$ 1,279	$ 1,414	$ 1,537	$ 2,544
Revenue year-to-year change (as a percent)	(0.60%)		(1.10%)
Pre-tax income year-to-year change (as a percent)	(9.50%)		(39.60%)
Pre-tax income margin (as a percent)	14.20%	15.60%	8.70%	14.30%
Business Segments | Systems
Segment Information
Revenue	$ 2,156	$ 2,730	$ 4,044	$ 5,044
Pre-tax income/(loss) from continuing operations	$ 229	$ 538	$ 218	$ 800
Revenue year-to-year change (as a percent)	(21.00%)		(19.80%)
Pre-tax income year-to-year change (as a percent)	(57.50%)		(72.70%)
Pre-tax income margin (as a percent)	10.60%	19.70%	5.40%	15.90%
Business Segments | Global Financing
Segment Information
Revenue	$ 926	$ 1,182	$ 1,822	$ 2,229
Pre-tax income/(loss) from continuing operations	$ 467	$ 613	$ 853	$ 1,128
Revenue year-to-year change (as a percent)	(21.70%)		(18.30%)
Pre-tax income year-to-year change (as a percent)	(23.80%)		(24.40%)
Pre-tax income margin (as a percent)	50.50%	51.90%	46.80%	50.60%
Internal transactions
Segment Information
Revenue	$ (1,561)	$ (1,728)	$ (3,206)	$ (3,419)
Pre-tax income/(loss) from continuing operations	(334)	(473)	(689)	(813)
Internal transactions | Cognitive Solutions
Segment Information
Revenue	(594)	(532)	(1,262)	(1,167)
Internal transactions | Global Business Services
Segment Information
Revenue	(103)	(130)	(216)	(261)
Internal transactions | Technology Services & Cloud Platforms
Segment Information
Revenue	(156)	(173)	(321)	(339)
Internal transactions | Systems
Segment Information
Revenue	(206)	(189)	(418)	(362)
Internal transactions | Global Financing
Segment Information
Revenue	$ (502)	$ (704)	$ (988)	$ (1,290)