Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Revenue by Major Products/Service Offerings
Total Revenue	$ 17,560	$ 18,028	$ 53,253	$ 55,370	$ 77,000
External Revenue
Revenue by Major Products/Service Offerings
Total Revenue	17,510	17,886	53,090	54,317
Other
Revenue by Major Products/Service Offerings
Revenue	50	142	163	1,053
Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	5,553	5,201	16,540	15,731
Total Revenue	5,553	5,201	16,540	15,731
Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	3,965	4,160	11,992	12,513
Total Revenue	3,965	4,160	11,992	12,513
Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	6,462	6,700	19,245	20,412
Total Revenue	6,462	6,700	19,245	20,412
Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,257	1,481	4,477	4,562
Total Revenue	1,257	1,481	4,477	4,562
Global Financing | External Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	273	343	837	1,100
Total Revenue	273	343	837	1,100
Cognitive Applications | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,317	1,305	3,745	3,914
Cloud & Data Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	2,775	2,308	8,108	6,398
Transaction Processing Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,461	1,589	4,687	5,419
Consulting | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,966	2,016	5,973	6,037
Application Management | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,758	1,897	5,332	5,724
Global Process Services | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	240	247	687	752
Infrastructure & Cloud Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	4,933	5,071	14,663	15,454
Technology Support Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,528	1,629	4,582	4,958
Systems Hardware | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	919	1,117	3,404	3,358
Operating Systems Software | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	$ 338	$ 364	$ 1,074	$ 1,204