RETIREMENT-RELATED BENEFITS - Defined Benefit Plans (Details)	12 Months Ended
Dec. 31, 2017
U.S. \| Pension Plans \| Personal Pension Plan (PPP)
Defined Benefit Plans
Period used in final pay formula that determines benefits	5 years

This element represents the period used in the final pay formula that determines the defined benefit based on salary, years of service, mortality and other participant-specific factors.

No definition available.

Name:	ibm_DefinedBenefitPlanPeriodUsedInFinalPayFormulaThatDeterminesBenefits
Namespace Prefix:	ibm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration