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Non-Marketable Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Carrying value of the Company's non-marketable investments	$ 3.7forr_CostAndEquityMethodInvestments		$ 3.8forr_CostAndEquityMethodInvestments
Book value of investment	0.6us-gaap_CostMethodInvestments		0.7us-gaap_CostMethodInvestments
Gross distributions received from funds	$ 0.1forr_CostMethodInvestmentOwnedIncomeReceivedFromFundDuringPeriod	$ 1.4forr_CostMethodInvestmentOwnedIncomeReceivedFromFundDuringPeriod
Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Ownership interest of Company	5.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_RangeAxis = us-gaap_MinimumMember

X

- Definition

Carrying amount of investments in equity method investees and the carrying amount, after adjustment, of cost-method investments.

+ References

No definition available.

+ Details

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Namespace Prefix:	forr_
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Balance Type:	debit
Period Type:	instant

X

- Definition

Gross income received from cost method investments funds during the period.

+ References

No definition available.

+ Details

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

X

- Definition

Amount, after adjustment, of cost-method investment. Adjustments include, but are not limited to, dividends received in excess of earnings after date of investment that are considered a return of investment and other than temporary impairments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 325

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6872867&loc=d3e40691-111596



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 325

-SubTopic 20

-Section 35

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=29638308&loc=d3e40346-111594

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

X

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

+ Details

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- Details

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- Details

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Namespace Prefix:
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