RENTAL EXPENSE AND LEASE COMMITMENTS - Commitments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Operating lease commitments
Gross minimum rental commitments (including vacant space below) for 2018	$ 1,614
Gross minimum rental commitments (including vacant space below) for 2019	1,453
Gross minimum rental commitments (including vacant space below) for 2020	1,143
Gross minimum rental commitments (including vacant space below) for 2021	829
Gross minimum rental commitments (including vacant space below) for 2022	633
Gross minimum rental commitments (including vacant space below) beyond 2022	896
Vacant space for 2018	32
Vacant space for 2019	21
Vacant space for 2020	15
Vacant space for 2021	11
Vacant space for 2022	7
Vacant space beyond 2022	2
Sublease income commitments for 2018	13
Sublease income commitments for 2019	7
Sublease income commitments for 2020	4
Sublease income commitments for 2021	2
Sublease income commitments for 2022	2
Capital lease commitments
Capital lease commitments for 2018	3
Capital lease commitments for 2019	2
Capital lease commitments for 2020	2
Capital lease commitments for 2021	2
Capital lease commitments for 2022	$ 1

Amount of future rental payments receivable on noncancelable subleasing arrangements due within one year.

No definition available.

Name:	ibm_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsCurrent
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of future rental payments receivable on noncancelable subleasing arrangements due within five years.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsWithinFiveYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of future rental payments receivable on noncancelable subleasing arrangements due within four years.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsWithinFourYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of future rental payments receivable on noncancelable subleasing arrangements due within three years.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsWithinThreeYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of future rental payments receivable on noncancelable subleasing arrangements due within two years.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsWithinTwoYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Vacant space, associated with infrastructure reductions and restructuring actions, rental payments within one year of the balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesVacantSpaceFutureMinimumPaymentsDueCurrent
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Vacant space, associated with infrastructure reductions and restructuring actions, rental payments within the fifth year of the balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesVacantSpaceFutureMinimumPaymentsDueInFiveYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Vacant space, associated with infrastructure reductions and restructuring actions, rental payments within the fourth year of the balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesVacantSpaceFutureMinimumPaymentsDueInFourYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Vacant space, associated with infrastructure reductions and restructuring actions, rental payments within the third year of the balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesVacantSpaceFutureMinimumPaymentsDueInThreeYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Vacant space, associated with infrastructure reductions and restructuring actions, rental payments within the second year of the balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesVacantSpaceFutureMinimumPaymentsDueInTwoYears
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Vacant space, associated with infrastructure reductions and restructuring actions, rental payments within, due after the fifth year from the balance sheet date on leases defined as operating.

+ References

No definition available.

+ Details

Name:	ibm_OperatingLeasesVacantSpaceFutureMinimumPaymentsDueThereafter
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84164817&loc=d3e45280-112737

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueInFiveYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueInFourYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueInThreeYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum lease payments for capital leases due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
-URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 55
-Paragraph 40
-Subparagraph (Note 3)
-URI http://asc.fasb.org/extlink&oid=82846649&loc=d3e38371-112697

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant