Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue by Major Products/Service Offerings
Total Revenue	$ 20,003	$ 19,289	$ 39,075	$ 37,443
Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	19,958	19,224	38,961	37,307
Other revenue
Revenue by Major Products/Service Offerings
Total Revenue	45	65	114	136
Cognitive Solutions | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	4,580	4,559	8,879	8,621
Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	4,192	4,097	8,365	8,103
Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	8,615	8,406	17,240	16,622
Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	2,177	1,747	3,676	3,142
Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	394	$ 415	799	$ 819
Solutions Software
Revenue by Major Products/Service Offerings
Total Revenue	3,214		6,171
Solutions Software | Cognitive Solutions | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	3,214		6,171
Transaction Processing Software
Revenue by Major Products/Service Offerings
Total Revenue	1,366		2,708
Transaction Processing Software | Cognitive Solutions | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,366		2,708
Consulting
Revenue by Major Products/Service Offerings
Total Revenue	1,931		3,798
Consulting | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,931		3,798
Global Process Services
Revenue by Major Products/Service Offerings
Total Revenue	315		620
Global Process Services | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	315		620
Application Management
Revenue by Major Products/Service Offerings
Total Revenue	1,946		3,948
Application Management | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,946		3,948
Infrastructure Services
Revenue by Major Products/Service Offerings
Total Revenue	5,768		11,593
Infrastructure Services | Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	5,768		11,593
Technical Support Services
Revenue by Major Products/Service Offerings
Total Revenue	1,750		3,531
Technical Support Services | Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,750		3,531
Integration Software
Revenue by Major Products/Service Offerings
Total Revenue	1,097		2,116
Integration Software | Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,097		2,116
Systems Hardware
Revenue by Major Products/Service Offerings
Total Revenue	1,756		2,848
Systems Hardware | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,756		2,848
Operating Systems Software
Revenue by Major Products/Service Offerings
Total Revenue	421		828
Operating Systems Software | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	421		828
Global Financing
Revenue by Major Products/Service Offerings
Total Revenue	394		799
Global Financing | Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	394		799
Other Revenue
Revenue by Major Products/Service Offerings
Total Revenue	45		114
Other Revenue | Other revenue
Revenue by Major Products/Service Offerings
Revenue	$ 45		$ 114