Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Revenue by Major Products/Service Offerings
Total Revenue	$ 18,123	$ 19,161	$ 35,694	$ 37,342	$ 77,000
External Revenue
Revenue by Major Products/Service Offerings
Total Revenue	18,072	18,701	35,581	36,432
Other
Revenue by Major Products/Service Offerings
Revenue	50	460	113	911
Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	5,748	5,563	10,987	10,530
Total Revenue	5,748	5,563	10,987	10,530
Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	3,890	4,197	8,027	8,353
Total Revenue	3,890	4,197	8,027	8,353
Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	6,316	6,837	12,783	13,711
Total Revenue	6,316	6,837	12,783	13,711
Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,852	1,753	3,220	3,081
Total Revenue	1,852	1,753	3,220	3,081
Global Financing | External Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	265	351	564	757
Total Revenue	265	351	564	757
Cognitive Applications | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,246	1,372	2,428	2,609
Cloud & Data Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	2,797	2,173	5,333	4,090
Transaction Processing Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,706	2,018	3,226	3,830
Consulting | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,936	2,020	4,007	4,021
Application Management | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,734	1,919	3,573	3,827
Global Process Services | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	221	258	447	505
Infrastructure & Cloud Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	4,813	5,174	9,729	10,383
Technology Support Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,503	1,663	3,054	3,328
Systems Hardware | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,488	1,328	2,484	2,241
Operating Systems Software | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	$ 365	$ 425	$ 736	$ 840