Lessee, operating lease, lease not yet commenced, undiscounted lease payments.

No definition available.

Name:	forr_LesseeOperatingLeaseLeaseNotYetCommencedUndiscountedLeasePayments
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents a receivable for an incentive or inducement contractually stipulated between parties to a lease whereby the lessor has committed to provide the entity (lessee) with a cash payment as inducement to enter the lease.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=77902458&loc=d3e40019-112707

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=77902458&loc=d3e40010-112707

+ Details

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

- Definition

Term of lessee's operating lease not yet commenced, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of loss from impairment of right-of-use asset from operating lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 25
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=77888369&loc=SL77918431-209957

+ Details

Name:	us-gaap_OperatingLeaseImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=forr_OfficeSpaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201602Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=forr_SiriusDecisionsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=forr_FacilityLeaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Leases - Additional Information (Detail) - USD ($)	3 Months Ended
Leases - Additional Information (Detail) - USD ($)	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Lessee Lease Description [Line Items]
Lessee, operating lease, lease not yet commenced, undiscounted lease payments			$ 13,900,000
Lessee, operating lease, lease not yet commenced, term of contract			10 years
Office Space [Member] \| Scenario, Forecast [Member] \| Minimum [Member]
Lessee Lease Description [Line Items]
Lease incentive receivable		$ 0
Office Space [Member] \| Scenario, Forecast [Member] \| Maximum [Member]
Lessee Lease Description [Line Items]
Lease incentive receivable		$ 3,500,000
Topic 842 [Member] \| SiriusDecisions, Inc [Member] \| Facility Lease [Member]
Lessee Lease Description [Line Items]
Operating lease right-of-use asset impairments	$ 1,400,000