Taxes - Tax Rate Reconciliation Narrative (Details) - USD ($) $ in Billions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective income tax rate reconciliation, additional disclosures
U.S. corporate tax rate	21.00%	21.00%	35.00%
Measurement period adjustments recognized related to U.S. tax reform	$ 0.1	$ 2.0	$ 5.5
Annual increase (decrease) in effective income tax rate	(15.90%)
Decrease related to U.S. tax reform	(16.50%)
Decrease related to intercompany payments	(3.40%)
Increase related to domestic and foreign audit activity	4.40%

Year-to-year increase (decrease) in the continuing operations effective income tax rate.

No definition available.

Name:	ibm_IncreaseDecreaseInContinuingOperationsEffectiveIncomeTaxRate
Namespace Prefix:	ibm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Year-to-year increase (decrease) in the continuing operations effective income tax rate related to audit activity.

+ References

No definition available.

+ Details

Name:	ibm_IncreaseDecreaseInContinuingOperationsEffectiveIncomeTaxRateAuditActivity
Namespace Prefix:	ibm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Year-to-year increase (decrease) in the continuing operations effective income tax rate related to intercompany payments.

+ References

No definition available.

+ Details

Name:	ibm_IncreaseDecreaseInContinuingOperationsEffectiveIncomeTaxRateIntercompanyPayments
Namespace Prefix:	ibm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Year-to-year increase (decrease) in the continuing operations effective income tax rate related to the total of all provisions of the Tax Cuts and Jobs Act of 2017.

+ References

No definition available.

+ Details

Name:	ibm_IncreaseDecreaseInContinuingOperationsEffectiveIncomeTaxRateTaxCutsAndJobsActOf2017TotalImpact
Namespace Prefix:	ibm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I)
-URI http://asc.fasb.org/extlink&oid=116825942&loc=d3e330036-122817

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of income tax expense (benefit) for measurement period adjustment from accounting for tax effect pursuant to Tax Cuts and Jobs Act.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 2A
-Subparagraph (SAB Topic 5.EE.Q2(f))
-URI http://asc.fasb.org/extlink&oid=116825942&loc=SL116722634-122817

+ Details

Name:	us-gaap_TaxCutsAndJobsActOf2017MeasurementPeriodAdjustmentIncomeTaxExpenseBenefit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration