Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Amount of intangible assets, excluding goodwill, acquired at the acquisition date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121647850&loc=d3e4845-128472

+ Details

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

- Details

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

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TechnologyBasedIntangibleAssetsMember
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_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_OrderOrProductionBacklogMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TrademarksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Acquisitions - Summary of Allocation of Purchase Price to Fair Value of Assets Acquired and Liabilities Assumed (Parenthetical) (Detail) - USD ($) $ in Thousands	Jan. 03, 2019	Jul. 06, 2018
SiriusDecisions, Inc [Member]
Business Acquisition [Line Items]
Intangible assets	$ 115,000
Intangible assets, Useful life	8 years 4 months 24 days
SiriusDecisions, Inc [Member] \| Technology Asset Class [Member]
Business Acquisition [Line Items]
Intangible assets	$ 13,000
Intangible assets, Useful life	3 years 2 months 12 days
SiriusDecisions, Inc [Member] \| Technology Asset Class [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Intangible assets, Useful life	1 year
SiriusDecisions, Inc [Member] \| Technology Asset Class [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Intangible assets, Useful life	8 years
SiriusDecisions, Inc [Member] \| Backlog [Member]
Business Acquisition [Line Items]
Intangible assets	$ 13,000
Intangible assets, Useful life	2 years
SiriusDecisions, Inc [Member] \| Customer Relationships [Member]
Business Acquisition [Line Items]
Intangible assets	$ 77,000
Intangible assets, Useful life	9 years 3 months
SiriusDecisions, Inc [Member] \| Trademarks [Member]
Business Acquisition [Line Items]
Intangible assets	$ 12,000
Intangible assets, Useful life	15 years 6 months
FeedbackNow [Member]
Business Acquisition [Line Items]
Intangible assets		$ 4,780
Intangible assets, Useful life		4 years 9 months 18 days
FeedbackNow [Member] \| Technology Asset Class [Member]
Business Acquisition [Line Items]
Intangible assets		$ 3,000
Intangible assets, Useful life		6 years 6 months
FeedbackNow [Member] \| Customer Relationships [Member]
Business Acquisition [Line Items]
Intangible assets		$ 1,300
Intangible assets, Useful life		6 years 1 month 6 days
FeedbackNow [Member] \| Customer Relationships [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Intangible assets, Useful life		4 years 6 months
FeedbackNow [Member] \| Customer Relationships [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Intangible assets, Useful life		7 years 6 months
FeedbackNow [Member] \| Trademarks [Member]
Business Acquisition [Line Items]
Intangible assets		$ 500
Intangible assets, Useful life		8 years 6 months