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Summary of Significant Accounting Policies (Details 4) | 12 Months Ended
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Dec. 31, 2012
Technology [Member] |
Forrester provides for amortization of intangible assets, computed using an accelerated method according to the expected cash flows to be received from the underlying assets, over the respective lives |
Amortization of intangible assets, useful life | 7 years
Trademarks [Member] |
Forrester provides for amortization of intangible assets, computed using an accelerated method according to the expected cash flows to be received from the underlying assets, over the respective lives |
Amortization of intangible assets, useful life | 1 year
Backlog [Member] |
Forrester provides for amortization of intangible assets, computed using an accelerated method according to the expected cash flows to be received from the underlying assets, over the respective lives |
Amortization of intangible assets, useful life | 1 year
Maximum [Member] | Customer relationships [Member] |
Forrester provides for amortization of intangible assets, computed using an accelerated method according to the expected cash flows to be received from the underlying assets, over the respective lives |
Amortization of intangible assets, useful life | 11 years
Maximum [Member] | Research content [Member] |
Forrester provides for amortization of intangible assets, computed using an accelerated method according to the expected cash flows to be received from the underlying assets, over the respective lives |
Amortization of intangible assets, useful life | 2 years
Minimum [Member] | Customer relationships [Member] |
Forrester provides for amortization of intangible assets, computed using an accelerated method according to the expected cash flows to be received from the underlying assets, over the respective lives |
Amortization of intangible assets, useful life | 5 years
Minimum [Member] | Research content [Member] |
Forrester provides for amortization of intangible assets, computed using an accelerated method according to the expected cash flows to be received from the underlying assets, over the respective lives |
Amortization of intangible assets, useful life | 1 year

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- Details
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Namespace Prefix: | forr_
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Balance Type: | na
Period Type: | duration

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- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration