Financing Receivables - Past Due (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Past Due Financing Receivable
Amortized cost/Recorded investment	$ 17,534	$ 22,421
Total Client Financing Receivables Portfolio Segment
Past Due Financing Receivable
Amortized cost/Recorded investment	15,411	18,590
Amortized cost/Recorded Investment Not Accruing	229	235
Impaired financing receivables individually evaluated for impairment with related allowance		191
Impaired financing receivables, related allowance	170	171
Total Client Financing Receivables Portfolio Segment \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days	416	465
Amortized cost/Recorded Investment > 90 Days and Accruing	206	253
Billed Invoices > 90 Days and Accruing	41	29
Total Client Financing Receivables Portfolio Segment \| Americas
Past Due Financing Receivable
Amortized cost/Recorded investment	7,631	10,144
Amortized cost/Recorded Investment Not Accruing	102
Total Client Financing Receivables Portfolio Segment \| Americas \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days	257
Amortized cost/Recorded Investment > 90 Days and Accruing	166
Billed Invoices > 90 Days and Accruing	31
Total Client Financing Receivables Portfolio Segment \| EMEA
Past Due Financing Receivable
Amortized cost/Recorded investment	4,813	5,087
Amortized cost/Recorded Investment Not Accruing	93
Total Client Financing Receivables Portfolio Segment \| EMEA \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days	99
Amortized cost/Recorded Investment > 90 Days and Accruing	13
Billed Invoices > 90 Days and Accruing	5
Total Client Financing Receivables Portfolio Segment \| Asia Pacific
Past Due Financing Receivable
Amortized cost/Recorded investment	2,968	3,359
Amortized cost/Recorded Investment Not Accruing	34
Total Client Financing Receivables Portfolio Segment \| Asia Pacific \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days	60
Amortized cost/Recorded Investment > 90 Days and Accruing	27
Billed Invoices > 90 Days and Accruing	6
Lease receivables
Past Due Financing Receivable
Amortized cost/Recorded investment	4,292	5,567
Amortized cost/Recorded Investment Not Accruing		69
Lease receivables \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		234
Amortized cost/Recorded Investment > 90 Days and Accruing		168
Billed Invoices > 90 Days and Accruing		14
Lease receivables \| Americas
Past Due Financing Receivable
Amortized cost/Recorded investment		3,419
Amortized cost/Recorded Investment Not Accruing		41
Lease receivables \| Americas \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		187
Amortized cost/Recorded Investment > 90 Days and Accruing		147
Billed Invoices > 90 Days and Accruing		11
Lease receivables \| EMEA
Past Due Financing Receivable
Amortized cost/Recorded investment		1,186
Amortized cost/Recorded Investment Not Accruing		17
Lease receivables \| EMEA \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		28
Amortized cost/Recorded Investment > 90 Days and Accruing		13
Billed Invoices > 90 Days and Accruing		2
Lease receivables \| Asia Pacific
Past Due Financing Receivable
Amortized cost/Recorded investment		963
Amortized cost/Recorded Investment Not Accruing		11
Lease receivables \| Asia Pacific \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		19
Amortized cost/Recorded Investment > 90 Days and Accruing		7
Billed Invoices > 90 Days and Accruing		1
Loan receivables
Past Due Financing Receivable
Amortized cost/Recorded investment	$ 11,119	13,022
Amortized cost/Recorded Investment Not Accruing		166
Loan receivables \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		231
Amortized cost/Recorded Investment > 90 Days and Accruing		85
Billed Invoices > 90 Days and Accruing		15
Loan receivables \| Americas
Past Due Financing Receivable
Amortized cost/Recorded investment		6,726
Amortized cost/Recorded Investment Not Accruing		72
Loan receivables \| Americas \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		127
Amortized cost/Recorded Investment > 90 Days and Accruing		71
Billed Invoices > 90 Days and Accruing		11
Loan receivables \| EMEA
Past Due Financing Receivable
Amortized cost/Recorded investment		3,901
Amortized cost/Recorded Investment Not Accruing		72
Loan receivables \| EMEA \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		77
Amortized cost/Recorded Investment > 90 Days and Accruing		8
Billed Invoices > 90 Days and Accruing		3
Loan receivables \| Asia Pacific
Past Due Financing Receivable
Amortized cost/Recorded investment		2,395
Amortized cost/Recorded Investment Not Accruing		21
Loan receivables \| Asia Pacific \| Total Past Due > 90 days
Past Due Financing Receivable
Amortized cost/Recorded Investment > 90 Days		26
Amortized cost/Recorded Investment > 90 Days and Accruing		6
Billed Invoices > 90 Days and Accruing		$ 2

Billed invoices in financing receivables that are 90 days or more past due and still accruing.

No definition available.

Name:	ibm_FinancingReceivableBilledInvoices90DaysPastDueAndStillAccruing
Namespace Prefix:	ibm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable on nonaccrual status.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5111-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable past due.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919253-210447

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953401-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=ibm_ClientFinancingReceivablesPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=srt_AmericasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=us-gaap_EMEAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=srt_AsiaPacificMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_FinanceLeasesPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: