Retirement-Related Benefits - Contributions, Defined Contribution Plans (Details) - USD ($) $ in Billions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Retirement-Related Benefits
Cash contribution by employer to defined contribution plans	$ 1.0	$ 1.0
Estimated future employer contributions to defined contribution plans in next fiscal year	$ 1.0

Amount of employer's best estimate of contributions expected to be paid to defined contribution plans during the next fiscal year beginning after the date of the latest statement of financial position.

No definition available.

Name:	ibm_DefinedContributionPlansEstimatedFutureEmployerContributionsInNextFiscalYear
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration