Rental Expense and Lease Commitments - Expense (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement Disclosures
Rental expense	$ 1,944	$ 1,821	$ 1,508

Rental expense including amounts charged to inventories and fixed assets, excluding amounts previously reserved. Includes rental expense reflected in discontinued operations.

No definition available.

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