Segments - Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)
Segment Information
Number of business segments (in segments) | segment			5
Revenue (excluding other revenue not allocated to segments)	$ 19,145	$ 19,219	$ 57,926	$ 59,520
Revenue	19,226	19,280	58,149	59,682
Pre-tax income from continuing operations	3,263	3,621	7,345	10,846
Cognitive Solutions
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,235	4,052	12,889	12,616
Global Business Services
Segment Information
Revenue (excluding other revenue not allocated to segments)	4,191	4,206	12,578	12,869
Technology Services & Cloud Platforms
Segment Information
Revenue (excluding other revenue not allocated to segments)	8,748	8,541	26,029	25,993
Systems
Segment Information
Revenue (excluding other revenue not allocated to segments)	1,558	1,973	5,184	6,656
Global Financing
Segment Information
Revenue (excluding other revenue not allocated to segments)	412	447	1,245	1,386
Business Segments
Segment Information
Revenue	20,613	20,821	62,599	64,540
Pre-tax income from continuing operations	$ 3,897	$ 4,387	$ 9,636	$ 13,444
Revenue year-to-year change (as a percent)	(1.00%)		(3.00%)
Pre-tax income year-to-year change (as a percent)	(11.20%)		(28.30%)
Pre-tax income margin (as a percent)	18.90%	21.10%	15.40%	20.80%
Business Segments | Cognitive Solutions
Segment Information
Revenue	$ 4,902	$ 4,580	$ 14,818	$ 14,311
Pre-tax income from continuing operations	$ 1,574	$ 1,596	$ 4,039	$ 4,949
Revenue year-to-year change (as a percent)	7.00%		3.50%
Pre-tax income year-to-year change (as a percent)	(1.40%)		(18.40%)
Pre-tax income margin (as a percent)	32.10%	34.90%	27.30%	34.60%
Business Segments | Global Business Services
Segment Information
Revenue	$ 4,284	$ 4,326	$ 12,888	$ 13,249
Pre-tax income from continuing operations	$ 544	$ 664	$ 1,210	$ 1,895
Revenue year-to-year change (as a percent)	(1.00%)		(2.70%)
Pre-tax income year-to-year change (as a percent)	(18.00%)		(36.20%)
Pre-tax income margin (as a percent)	12.70%	15.40%	9.40%	14.30%
Business Segments | Technology Services & Cloud Platforms
Segment Information
Revenue	$ 8,929	$ 8,702	$ 26,530	$ 26,493
Pre-tax income from continuing operations	$ 1,288	$ 1,317	$ 2,825	$ 3,861
Revenue year-to-year change (as a percent)	2.60%		0.10%
Pre-tax income year-to-year change (as a percent)	(2.20%)		(26.80%)
Pre-tax income margin (as a percent)	14.40%	15.10%	10.60%	14.60%
Business Segments | Systems
Segment Information
Revenue	$ 1,734	$ 2,182	$ 5,778	$ 7,226
Pre-tax income from continuing operations	$ 136	$ 248	$ 354	$ 1,048
Revenue year-to-year change (as a percent)	(20.50%)		(20.00%)
Pre-tax income year-to-year change (as a percent)	(45.30%)		(66.20%)
Pre-tax income margin (as a percent)	7.80%	11.40%	6.10%	14.50%
Business Segments | Global Financing
Segment Information
Revenue	$ 763	$ 1,031	$ 2,585	$ 3,261
Pre-tax income from continuing operations	$ 355	$ 562	$ 1,208	$ 1,690
Revenue year-to-year change (as a percent)	(26.00%)		(20.70%)
Pre-tax income year-to-year change (as a percent)	(36.80%)		(28.50%)
Pre-tax income margin (as a percent)	46.50%	54.50%	46.70%	51.80%
Internal transactions
Segment Information
Revenue	$ (1,468)	$ (1,601)	$ (4,673)	$ (5,020)
Pre-tax income from continuing operations	(185)	(362)	(873)	(1,175)
Internal transactions | Cognitive Solutions
Segment Information
Revenue	(667)	(528)	(1,929)	(1,695)
Internal transactions | Global Business Services
Segment Information
Revenue	(93)	(120)	(310)	(380)
Internal transactions | Technology Services & Cloud Platforms
Segment Information
Revenue	(180)	(161)	(501)	(500)
Internal transactions | Systems
Segment Information
Revenue	(176)	(209)	(594)	(571)
Internal transactions | Global Financing
Segment Information
Revenue	$ (352)	$ (584)	$ (1,340)	$ (1,874)