Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segments
Revenue	$ 19,161	$ 20,003	$ 37,342	$ 39,075
Pre-tax income/(loss) from continuing operations	2,768	2,776	4,651	3,911
Business Segments, Excluding Intersegment Revenue
Segments
Revenue	18,741	19,500	36,506	38,057
Business Segments, Excluding Intersegment Revenue | Cloud & Cognitive Software
Segments
Revenue	5,645	5,470	10,682	10,586
Business Segments, Excluding Intersegment Revenue | Global Business Services
Segments
Revenue	4,155	4,135	8,274	8,250
Business Segments, Excluding Intersegment Revenue | Global Technology Services
Segments
Revenue	6,837	7,325	13,711	14,746
Business Segments, Excluding Intersegment Revenue | Systems
Segments
Revenue	1,753	2,177	3,081	3,676
Business Segments, Excluding Intersegment Revenue | Global Financing
Segments
Revenue	351	394	757	799
Internal transactions
Segments
Revenue	(1,430)	(1,778)	(3,097)	(3,521)
Pre-tax income/(loss) from continuing operations	(60)	(308)	(149)	(497)
Internal transactions | Cloud & Cognitive Software
Segments
Revenue	(607)	(811)	(1,448)	(1,741)
Internal transactions | Global Business Services
Segments
Revenue	(69)	(83)	(143)	(172)
Internal transactions | Global Technology Services
Segments
Revenue	(302)	(169)	(591)	(310)
Internal transactions | Systems
Segments
Revenue	(171)	(242)	(334)	(395)
Internal transactions | Global Financing
Segments
Revenue	(281)	(473)	(581)	(902)
Business Segments
Segments
Revenue	20,170	21,278	39,603	41,578
Pre-tax income/(loss) from continuing operations	$ 2,836	$ 3,556	$ 5,280	$ 5,601
Revenue year-to-year change (as a percent)	(5.20%)		(4.70%)
Pre-tax income year-to-year change (as a percent)	(20.20%)		(5.70%)
Pre-tax income/(loss) margin (as a percent)	14.10%	16.70%	13.30%	13.50%
Business Segments | Cloud & Cognitive Software
Segments
Revenue	$ 6,252	$ 6,280	$ 12,131	$ 12,327
Pre-tax income/(loss) from continuing operations	$ 2,001	$ 2,029	$ 3,768	$ 3,709
Revenue year-to-year change (as a percent)	(0.50%)		(1.60%)
Pre-tax income year-to-year change (as a percent)	(1.40%)		1.60%
Pre-tax income/(loss) margin (as a percent)	32.00%	32.30%	31.10%	30.10%
Business Segments | Global Business Services
Segments
Revenue	$ 4,224	$ 4,218	$ 8,417	$ 8,422
Pre-tax income/(loss) from continuing operations	$ 300	$ 372	$ 615	$ 497
Revenue year-to-year change (as a percent)	0.10%		(0.10%)
Pre-tax income year-to-year change (as a percent)	(19.40%)		23.70%
Pre-tax income/(loss) margin (as a percent)	7.10%	8.80%	7.30%	5.90%
Business Segments | Global Technology Services
Segments
Revenue	$ 7,139	$ 7,494	$ 14,303	$ 15,055
Pre-tax income/(loss) from continuing operations	$ 235	$ 451	$ 510	$ 517
Revenue year-to-year change (as a percent)	(4.70%)		(5.00%)
Pre-tax income year-to-year change (as a percent)	(47.90%)		(1.40%)
Pre-tax income/(loss) margin (as a percent)	3.30%	6.00%	3.60%	3.40%
Business Segments | Systems
Segments
Revenue	$ 1,924	$ 2,419	$ 3,415	$ 4,072
Pre-tax income/(loss) from continuing operations	$ 61	$ 346	$ (141)	$ 143
Revenue year-to-year change (as a percent)	(20.50%)		(16.10%)
Pre-tax income year-to-year change (as a percent)	(82.30%)		(198.10%)
Pre-tax income/(loss) margin (as a percent)	3.20%	14.30%	(4.10%)	3.50%
Business Segments | Global Financing
Segments
Revenue	$ 632	$ 867	$ 1,338	$ 1,701
Pre-tax income/(loss) from continuing operations	$ 239	$ 357	$ 527	$ 734
Revenue year-to-year change (as a percent)	(27.10%)		(21.30%)
Pre-tax income year-to-year change (as a percent)	(33.10%)		(28.10%)
Pre-tax income/(loss) margin (as a percent)	37.80%	41.20%	39.40%	43.10%