Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment
Revenue	$ 20,003	$ 19,289	$ 39,075	$ 37,443
Pre-tax income/(loss) from continuing operations	2,776	2,443	3,911	3,867
Business Segments, Excluding Intersegment Revenue
Segment
Revenue	19,958	19,224	38,961	37,307
Business Segments, Excluding Intersegment Revenue | Cognitive Solutions
Segment
Revenue	4,580	4,559	8,879	8,621
Business Segments, Excluding Intersegment Revenue | Global Business Services
Segment
Revenue	4,192	4,097	8,365	8,103
Business Segments, Excluding Intersegment Revenue | Technology Services & Cloud Platforms
Segment
Revenue	8,615	8,406	17,240	16,622
Business Segments, Excluding Intersegment Revenue | Systems
Segment
Revenue	2,177	1,747	3,676	3,142
Business Segments, Excluding Intersegment Revenue | Global Financing
Segment
Revenue	394	415	799	819
Internal transactions
Segment
Revenue	(1,670)	(1,388)	(3,262)	(2,881)
Pre-tax income/(loss) from continuing operations	(291)	(161)	(509)	(388)
Internal transactions | Cognitive Solutions
Segment
Revenue	(703)	(655)	(1,483)	(1,371)
Internal transactions | Global Business Services
Segment
Revenue	(83)	(93)	(172)	(179)
Internal transactions | Technology Services & Cloud Platforms
Segment
Revenue	(169)	(173)	(310)	(333)
Internal transactions | Systems
Segment
Revenue	(242)	(177)	(395)	(344)
Internal transactions | Global Financing
Segment
Revenue	(473)	(290)	(902)	(653)
Business Segments
Segment
Revenue	21,628	20,612	42,222	40,188
Pre-tax income/(loss) from continuing operations	$ 3,728	$ 3,270	$ 5,816	$ 5,616
Revenue year-to-year change (as a percent)	4.90%		5.10%
Pre-tax income year-to-year change (as a percent)	14.00%		3.60%
Pre-tax income/(loss) margin (as a percent)	17.20%	15.90%	13.80%	14.00%
Business Segments | Cognitive Solutions
Segment
Revenue	$ 5,283	$ 5,214	$ 10,362	$ 9,992
Pre-tax income/(loss) from continuing operations	$ 1,756	$ 1,610	$ 3,089	$ 2,878
Revenue year-to-year change (as a percent)	1.30%		3.70%
Pre-tax income year-to-year change (as a percent)	9.10%		7.30%
Pre-tax income/(loss) margin (as a percent)	33.20%	30.90%	29.80%	28.80%
Business Segments | Global Business Services
Segment
Revenue	$ 4,275	$ 4,190	$ 8,538	$ 8,282
Pre-tax income/(loss) from continuing operations	$ 385	$ 312	$ 530	$ 593
Revenue year-to-year change (as a percent)	2.00%		3.10%
Pre-tax income year-to-year change (as a percent)	23.70%		(10.60%)
Pre-tax income/(loss) margin (as a percent)	9.00%	7.40%	6.20%	7.20%
Business Segments | Technology Services & Cloud Platforms
Segment
Revenue	$ 8,784	$ 8,579	$ 17,550	$ 16,955
Pre-tax income/(loss) from continuing operations	$ 883	$ 994	$ 1,320	$ 1,668
Revenue year-to-year change (as a percent)	2.40%		3.50%
Pre-tax income year-to-year change (as a percent)	(11.20%)		(20.90%)
Pre-tax income/(loss) margin (as a percent)	10.10%	11.60%	7.50%	9.80%
Business Segments | Systems
Segment
Revenue	$ 2,419	$ 1,924	$ 4,072	$ 3,486
Pre-tax income/(loss) from continuing operations	$ 346	$ 73	$ 143	$ (115)
Revenue year-to-year change (as a percent)	25.70%		16.80%
Pre-tax income year-to-year change (as a percent)	376.80%
Pre-tax income/(loss) margin (as a percent)	14.30%	3.80%	3.50%	(3.30%)
Business Segments | Global Financing
Segment
Revenue	$ 867	$ 705	$ 1,701	$ 1,473
Pre-tax income/(loss) from continuing operations	$ 357	$ 282	$ 734	$ 592
Revenue year-to-year change (as a percent)	23.00%		15.50%
Pre-tax income year-to-year change (as a percent)	26.80%		23.90%
Pre-tax income/(loss) margin (as a percent)	41.20%	40.00%	43.10%	40.20%