Acquisitions - Additional Information (Detail) $ in Thousands				12 Months Ended
	Jan. 03, 2019 USD ($) Employee	Jul. 06, 2018 USD ($)	Jun. 22, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill				$ 247,211	$ 243,895	$ 85,165
Cash contingent paid on achievement of certain employment conditions				200	100
Acquisition and integration costs				5,779	8,948	3,787
Sirius Decisions Segment [Member]
Business Acquisition [Line Items]
Goodwill	$ 142,500				142,584
Research Segment [Member]
Business Acquisition [Line Items]
Goodwill	$ 16,000			238,913	101,311	$ 72,647
SiriusDecisions, Inc [Member]
Business Acquisition [Line Items]
Revenue					79,300
Direct expenses including intangible amortization					103,900
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
Purchase price paid	246,801
Goodwill	$ 158,569
FeedbackNow [Member]
Business Acquisition [Line Items]
Percentage of issued and outstanding shares acquired		100.00%
Purchase price paid		$ 8,425		$ 2,700	$ 1,800
Goodwill		$ 9,513
Acquisition date		Jul. 06, 2018
Balance acquisition price payable period		2 years
SocialGlimpz Inc. [Member]
Business Acquisition [Line Items]
Purchase price paid			$ 1,300
Goodwill			$ 700
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
SocialGlimpz Inc. [Member] | Technology [Member]
Business Acquisition [Line Items]
Intangible assets			$ 600
Estimated useful life			5 years