Acquisitions - Additional Information (Detail)				12 Months Ended
	Jan. 03, 2019 USD ($) Employee	Jul. 06, 2018 USD ($)	Jun. 22, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 06, 2018 CHF (SFr)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill				$ 243,895,000	$ 85,165,000		$ 76,169,000
Cash contingent paid on achievement of certain employment conditions				100,000
Sirius Decisions Segment [Member]
Business Acquisition [Line Items]
Goodwill	$ 142,500,000			142,584,000
Research Segment [Member]
Business Acquisition [Line Items]
Goodwill	$ 16,000,000			101,311,000	$ 72,647,000		$ 73,731,000
SiriusDecisions, Inc [Member]
Business Acquisition [Line Items]
Revenue				79,300,000
Direct expenses including intangible amortization				103,900,000
SiriusDecisions, Inc [Member]
Business Acquisition [Line Items]
Percentage of issued and outstanding shares acquired	100.00%
Number of employees | Employee	350
Consideration paid	$ 245,000,000
Purchase price paid	246,801,000
Goodwill	158,569,000
Acquisition costs recognized				1,700,000
Additional purchase price payable based on acquired working capital	$ (1,259,000)
FeedbackNow [Member]
Business Acquisition [Line Items]
Percentage of issued and outstanding shares acquired		100.00%				100.00%
Purchase price paid		$ 8,425,000		500,000
Goodwill		$ 9,513,000
Acquisition date		Jul. 06, 2018
Acquisition price due and subject to indemnity provisions from seller		$ 1,485,000
Additional purchase price payable based on acquired working capital		$ 798,000		800,000
Balance acquisition price payable period		2 years
Maximum consideration payable based on the financial performance of acquired company		$ 2,500,000		$ 4,300,000		SFr 4,200,000
SocialGlimpz Inc. [Member]
Business Acquisition [Line Items]
Purchase price paid			$ 1,300,000
Goodwill			$ 700,000
Acquisition date			Jun. 22, 2018
Name of the business entity acquired				SocialGlimpz, Inc.
Description of acquired entity				On June 22, 2018, Forrester acquired substantially all of the assets of SocialGlimpz, Inc. (“GlimpzIt”), an artificial intelligence and machine-learning provider based in San Francisco. The acquisition is part of Forrester's plan to build a real-time CX cloud solution, integrating a range of inputs to help companies monitor and improve customer experience. Forrester intends to deploy the GlimpzIt technology to extend the analytics engine in Forrester’s planned real-time CX cloud.
Cash contingent to be paid on achievement of certain employment conditions			$ 300,000
Compensation expense to be recognized over related service period of employees			2 years
SocialGlimpz Inc. [Member] | Technology [Member]
Business Acquisition [Line Items]
Intangible assets			$ 600,000
Estimated useful life			5 years