Stockholders' Equity - Components of Stockholders' Equity (Parenthetical) (Detail) $ in Thousands	9 Months Ended
Sep. 30, 2020 USD ($)
Statement Of Stockholders Equity [Abstract]
Cumulative effect adjustment due to adoption of new accounting pronouncement tax	$ 61

Cumulative effect adjustment due to adoption of new accounting pronouncement tax.

No definition available.

Name:	forr_CumulativeEffectAdjustmentDueToAdoptionOfNewAccountingPronouncementTax
Namespace Prefix:	forr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration