Acquisitions - Additional Information (Detail)				3 Months Ended		6 Months Ended	9 Months Ended
	Jan. 03, 2019 USD ($) Employee	Jul. 06, 2018 USD ($)	Jun. 22, 2018 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 06, 2018 CHF (SFr)
Business Acquisition [Line Items]
Goodwill				$ 238,826,000			$ 238,826,000			$ 85,165,000
SiriusDecisions, Inc [Member]
Business Acquisition [Line Items]
Revenue				16,800,000			59,500,000
Direct expenses including intangible amortization				22,200,000			$ 79,200,000
SocialGlimpz Inc. [Member]
Business Acquisition [Line Items]
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 customer experience or 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.
Purchase price paid			$ 1,300,000
Goodwill			700,000
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
Cash contingent paid on achievement of certain employment conditions						$ 100,000
SocialGlimpz Inc. [Member] | Technology [Member]
Business Acquisition [Line Items]
Intangible assets			$ 600,000
Estimated useful life			5 years
FeedbackNow [Member]
Business Acquisition [Line Items]
Acquisition date		Jul. 06, 2018
Purchase price paid		$ 8,400,000					$ 500,000
Percentage of issued and outstanding shares acquired		100.00%									100.00%
Acquisition price due and subject to indemnity provisions from seller		$ 1,500,000
Additional purchase price paid							800,000
Maximum consideration payable based on the financial performance of acquired company		2,500,000		4,300,000			4,300,000				SFr 4,200,000
Additional consideration payable to sellers based on financial performance				$ 1,800,000			1,800,000
Business acquisition undiscounted amounts could be payable, minimum		0
Contingent purchase price		$ 3,400,000
FeedbackNow [Member] | Scenario, Forecast [Member] | Maximum [Member]
Business Acquisition [Line Items]
Additional consideration payable to sellers based on financial performance								$ 2,500,000	$ 1,800,000
SiriusDecisions, Inc [Member]
Business Acquisition [Line Items]
Purchase price paid	$ 246,801,000
Goodwill	$ 154,906,000
Percentage of issued and outstanding shares acquired	100.00%
Number of employees | Employee	350
Consideration paid	$ 245,000,000
Acquisition costs recognized					$ 1,700,000		$ 1,700,000