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Acquisitions - Additional Information (Detail) - USD ($) $ in Thousands		6 Months Ended
	Jun. 22, 2018	Jun. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Purchase price paid		$ 1,289
Goodwill		$ 76,551	$ 76,169
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
Goodwill	1,000
Intangible assets	$ 300