Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue by Major Products/Service Offerings
Total Revenue	$ 18,182	$ 19,072
Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	17,765	18,557
Other
Revenue by Major Products/Service Offerings
Total Revenue	417	515
Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	5,037	5,116
Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	4,119	4,115
Global Technology Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	6,875	7,421
Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	1,328	1,500
Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	406	405
Cognitive Applications
Revenue by Major Products/Service Offerings
Total Revenue	1,308	1,286
Cognitive Applications | Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,308	1,286
Cloud & Data Platforms
Revenue by Major Products/Service Offerings
Total Revenue	1,917	1,950
Cloud & Data Platforms | Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,917	1,950
Transaction Processing Platforms
Revenue by Major Products/Service Offerings
Total Revenue	1,812	1,880
Transaction Processing Platforms | Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,812	1,880
Consulting
Revenue by Major Products/Service Offerings
Total Revenue	1,964	1,867
Consulting | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,964	1,867
Global Process Services
Revenue by Major Products/Service Offerings
Total Revenue	247	246
Global Process Services | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	247	246
Application Management
Revenue by Major Products/Service Offerings
Total Revenue	1,908	2,002
Application Management | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,908	2,002
Infrastructure & Cloud Services
Revenue by Major Products/Service Offerings
Total Revenue	5,209	5,639
Infrastructure & Cloud Services | Global Technology Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	5,209	5,639
Technology Support Services
Revenue by Major Products/Service Offerings
Total Revenue	1,665	1,782
Technology Support Services | Global Technology Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,665	1,782
Systems Hardware
Revenue by Major Products/Service Offerings
Total Revenue	914	1,093
Systems Hardware | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	914	1,093
Operating Systems Software
Revenue by Major Products/Service Offerings
Total Revenue	414	407
Operating Systems Software | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	414	407
Global Financing
Revenue by Major Products/Service Offerings
Total Revenue	406	405
Global Financing | Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	406	405
Other Revenue
Revenue by Major Products/Service Offerings
Total Revenue	417	515
Other Revenue | Other
Revenue by Major Products/Service Offerings
Revenue	$ 417	$ 515