Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue by Major Products/Service Offerings
Total Revenue	$ 17,730	$ 17,571
External Revenue
Revenue by Major Products/Service Offerings
Total Revenue	17,707	17,508
Other
Revenue by Major Products/Service Offerings
Revenue	23	62
Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	5,437	5,238
Total Revenue	5,437	5,238
Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	4,234	4,136
Total Revenue	4,234	4,136
Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	6,370	6,467
Total Revenue	6,370	6,467
Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,427	1,368
Total Revenue	1,427	1,368
Global Financing | External Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	240	299
Total Revenue	240	299
Cloud & Data Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	2,866	2,536
Cognitive Applications | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,233	1,182
Transaction Processing Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,338	1,520
Consulting | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	2,189	2,071
Application Management | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,770	1,840
Global Process Services | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	274	225
Infrastructure & Cloud Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	4,853	4,916
Technology Support Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,517	1,550
Systems Hardware | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,114	997
Operating Systems Software | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	$ 313	$ 371