Segments - Results of Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segments
Revenue	$ 18,182	$ 19,072
Pre-tax income/(loss) from continuing operations	1,883	1,136
Business Segments, Excluding Intersegment Revenue
Segments
Revenue	17,765	18,557
Business Segments, Excluding Intersegment Revenue \| Cloud & Cognitive Software
Segments
Revenue	5,037	5,116
Business Segments, Excluding Intersegment Revenue \| Global Business Services
Segments
Revenue	4,119	4,115
Business Segments, Excluding Intersegment Revenue \| Global Technology Services
Segments
Revenue	6,875	7,421
Business Segments, Excluding Intersegment Revenue \| Systems
Segments
Revenue	1,328	1,500
Business Segments, Excluding Intersegment Revenue \| Global Financing
Segments
Revenue	406	405
Internal transactions
Segments
Revenue	(1,668)	(1,743)
Pre-tax income/(loss) from continuing operations	(89)	(190)
Internal transactions \| Cloud & Cognitive Software
Segments
Revenue	(841)	(931)
Internal transactions \| Global Business Services
Segments
Revenue	(74)	(89)
Internal transactions \| Global Technology Services
Segments
Revenue	(290)	(141)
Internal transactions \| Systems
Segments
Revenue	(163)	(153)
Internal transactions \| Global Financing
Segments
Revenue	(300)	(429)
Business Segments
Segments
Revenue	19,433	20,299
Pre-tax income/(loss) from continuing operations	$ 2,444	$ 2,045
Revenue year-to-year change (as a percent)	(4.30%)
Pre-tax income year-to-year change (as a percent)	19.50%
Pre-tax income/(loss) margin (as a percent)	12.60%	10.10%
Business Segments \| Cloud & Cognitive Software
Segments
Revenue	$ 5,879	$ 6,047
Pre-tax income/(loss) from continuing operations	$ 1,767	$ 1,680
Revenue year-to-year change (as a percent)	(2.80%)
Pre-tax income year-to-year change (as a percent)	5.20%
Pre-tax income/(loss) margin (as a percent)	30.10%	27.80%
Business Segments \| Global Business Services
Segments
Revenue	$ 4,193	$ 4,204
Pre-tax income/(loss) from continuing operations	$ 315	$ 125
Revenue year-to-year change (as a percent)	(0.30%)
Pre-tax income year-to-year change (as a percent)	152.10%
Pre-tax income/(loss) margin (as a percent)	7.50%	3.00%
Business Segments \| Global Technology Services
Segments
Revenue	$ 7,164	$ 7,562
Pre-tax income/(loss) from continuing operations	$ 275	$ 66
Revenue year-to-year change (as a percent)	(5.30%)
Pre-tax income year-to-year change (as a percent)	315.10%
Pre-tax income/(loss) margin (as a percent)	3.80%	0.90%
Business Segments \| Systems
Segments
Revenue	$ 1,491	$ 1,653
Pre-tax income/(loss) from continuing operations	$ (202)	$ (203)
Revenue year-to-year change (as a percent)	(9.80%)
Pre-tax income year-to-year change (as a percent)	(0.40%)
Pre-tax income/(loss) margin (as a percent)	(13.50%)	(12.30%)
Business Segments \| Global Financing
Segments
Revenue	$ 706	$ 834
Pre-tax income/(loss) from continuing operations	$ 288	$ 377
Revenue year-to-year change (as a percent)	(15.40%)
Pre-tax income year-to-year change (as a percent)	(23.40%)
Pre-tax income/(loss) margin (as a percent)	40.80%	45.10%

The percentage change from year to year in the pre-tax income for segment reporting purposes.

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The pre-tax income margin percentage for segment reporting purposes.

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The percentage change from year to year in the revenue for segment reporting purposes.

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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