Revenue Recognition - Disaggregation of Revenue by Major Products/Service Offerings (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue by Major Products/Service Offerings
Total Revenue	$ 18,745	$ 18,123	$ 36,474	$ 35,694
External Revenue
Revenue by Major Products/Service Offerings
Total Revenue	18,740	18,072	36,447	35,581
Other revenue
Revenue by Major Products/Service Offerings
Revenue	5	50	28	113
Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	6,098	5,748	11,534	10,987
Total Revenue	6,098	5,748	11,534	10,987
Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	4,341	3,890	8,575	8,027
Total Revenue	4,341	3,890	8,575	8,027
Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	6,342	6,316	12,712	12,783
Total Revenue	6,342	6,316	12,712	12,783
Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,717	1,852	3,144	3,220
Total Revenue	1,717	1,852	3,144	3,220
Global Financing | External Revenue
Revenue by Major Products/Service Offerings
Financial Services Revenue	242	265	482	564
Total Revenue	242	265	482	564
Cloud & Data Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	3,120	2,797	5,986	5,333
Cognitive Applications | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,391	1,246	2,624	2,428
Transaction Processing Platforms | Cloud & Cognitive Software | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,587	1,706	2,925	3,226
Consulting | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	2,236	1,936	4,425	4,007
Application Management | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,821	1,734	3,591	3,573
Global Process Services | Global Business Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	284	221	559	447
Infrastructure & Cloud Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	4,836	4,813	9,689	9,729
Technology Support Services | Global Technology Services | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,506	1,503	3,023	3,054
Systems Hardware | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	1,384	1,488	2,498	2,484
Operating Systems Software | Systems | External Revenue
Revenue by Major Products/Service Offerings
Revenue	$ 333	$ 365	$ 646	$ 736