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Summary of Significant Accounting Policies (Details 4)	12 Months Ended
	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

X

- Details

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X

- 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

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