Fair Value Measurements - Summary of Changes in the Fair Value of Level 3 Contingent Consideration (Parenthetical) (Detail) $ in Millions	3 Months Ended
Dec. 31, 2020 USD ($)
Fair Value Disclosures [Abstract]
Consideration paid to sellers based on financial performance	$ 2.7

Business combinations additional consideration paid to sellers.

No definition available.

Name:	forr_BusinessCombinationsAdditionalConsiderationPaidToSellers
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration