Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue by Major Products/Service Offerings
Total Revenue	$ 18,028	$ 18,756	$ 55,370	$ 57,830
Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	17,921	18,262	54,426	56,319
Other
Revenue by Major Products/Service Offerings
Total Revenue	107	493	944	1,511
Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	5,280	4,962	15,962	15,548
Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	4,117	4,076	12,391	12,326
Global Technology Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	6,700	7,101	20,412	21,846
Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	1,481	1,736	4,562	5,412
Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	343	388	1,100	1,188
Cognitive Applications
Revenue by Major Products/Service Offerings
Revenue	1,383	1,324	4,145	4,023
Cognitive Applications | Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,383	1,324	4,145	4,023
Cloud & Data Platforms
Revenue by Major Products/Service Offerings
Revenue	2,308	1,968	6,398	5,997
Cloud & Data Platforms | Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	2,308	1,968	6,398	5,997
Transaction Processing Platforms
Revenue by Major Products/Service Offerings
Revenue	1,589	1,669	5,419	5,528
Transaction Processing Platforms | Cloud & Cognitive Software | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,589	1,669	5,419	5,528
Consulting
Revenue by Major Products/Service Offerings
Revenue	1,973	1,900	5,915	5,698
Consulting | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,973	1,900	5,915	5,698
Global Process Services
Revenue by Major Products/Service Offerings
Revenue	247	260	752	765
Global Process Services | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	247	260	752	765
Application Management
Revenue by Major Products/Service Offerings
Revenue	1,897	1,915	5,724	5,863
Application Management | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,897	1,915	5,724	5,863
Infrastructure & Cloud Services
Revenue by Major Products/Service Offerings
Revenue	5,071	5,393	15,454	16,608
Infrastructure & Cloud Services | Global Technology Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	5,071	5,393	15,454	16,608
Technology Support Services
Revenue by Major Products/Service Offerings
Revenue	1,629	1,707	4,958	5,239
Technology Support Services | Global Technology Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,629	1,707	4,958	5,239
Systems Hardware
Revenue by Major Products/Service Offerings
Revenue	1,117	1,339	3,358	4,187
Systems Hardware | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,117	1,339	3,358	4,187
Operating Systems Software
Revenue by Major Products/Service Offerings
Revenue	364	397	1,204	1,225
Operating Systems Software | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	364	397	1,204	1,225
Global Financing
Revenue by Major Products/Service Offerings
Financial Services Revenue	343	388	1,100	1,188
Global Financing | Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	343	388	1,100	1,188
Other Revenue
Revenue by Major Products/Service Offerings
Revenue	107	493	944	1,511
Other Revenue | Other
Revenue by Major Products/Service Offerings
Revenue	$ 107	$ 493	$ 944	$ 1,511