Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue by Major Products/Service Offerings
Total Revenue	$ 18,756	$ 19,153	$ 57,830	$ 56,597
Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	18,694	19,098	57,655	56,405
Other revenue
Revenue by Major Products/Service Offerings
Total Revenue	62	56	176	192
Cognitive Solutions | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	4,148	4,400	13,027	13,021
Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	4,130	4,093	12,495	12,196
Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	8,292	8,457	25,533	25,079
Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	1,736	1,721	5,412	4,863
Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Total Revenue	388	$ 427	1,188	$ 1,246
Solutions Software
Revenue by Major Products/Service Offerings
Total Revenue	2,910		9,080
Solutions Software | Cognitive Solutions | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	2,910		9,080
Transaction Processing Software
Revenue by Major Products/Service Offerings
Total Revenue	1,238		3,946
Transaction Processing Software | Cognitive Solutions | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,238		3,946
Consulting
Revenue by Major Products/Service Offerings
Total Revenue	1,900		5,698
Consulting | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,900		5,698
Global Process Services
Revenue by Major Products/Service Offerings
Total Revenue	314		934
Global Process Services | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	314		934
Application Management
Revenue by Major Products/Service Offerings
Total Revenue	1,915		5,863
Application Management | Global Business Services | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,915		5,863
Infrastructure Services
Revenue by Major Products/Service Offerings
Total Revenue	5,595		17,188
Infrastructure Services | Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	5,595		17,188
Technical Support Services
Revenue by Major Products/Service Offerings
Total Revenue	1,707		5,239
Technical Support Services | Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,707		5,239
Integration Software
Revenue by Major Products/Service Offerings
Total Revenue	990		3,106
Integration Software | Technology Services & Cloud Platforms | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	990		3,106
Systems Hardware
Revenue by Major Products/Service Offerings
Total Revenue	1,339		4,187
Systems Hardware | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	1,339		4,187
Operating Systems Software
Revenue by Major Products/Service Offerings
Total Revenue	397		1,225
Operating Systems Software | Systems | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Revenue	397		1,225
Global Financing
Revenue by Major Products/Service Offerings
Total Revenue	388		1,188
Global Financing | Global Financing | Business Segments, Excluding Intersegment Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	388		1,188
Other Revenue
Revenue by Major Products/Service Offerings
Total Revenue	62		176
Other Revenue | Other revenue
Revenue by Major Products/Service Offerings
Revenue	$ 62		$ 176