Interim Consolidated Financial Statements (Policies)	9 Months Ended
Interim Consolidated Financial Statements (Policies)	Sep. 30, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited interim consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for reporting on Form 10-Q. Accordingly, certain information and footnote disclosures required for complete financial statements are not included herein. The year-end balance sheet data was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States of America. It is recommended that these financial statements be read in conjunction with the consolidated financial statements and related notes that appear in the Forrester Research, Inc. (“Forrester”) Annual Report on Form 10-K for the year ended December 31, 2013. In the opinion of management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation of the financial position, results of operations, of comprehensive income and of cash flows as of the dates and for the periods presented have been included. The results of operations for the three and nine months ended September 30, 2014 may not be indicative of the results for the year ending December 31, 2014, or any other period. During the quarter ended March 31, 2014, the Company recorded $0.5 million of expenses for out-of-period corrections, of which $0.4 million related to depreciation and $0.1 million related to other immaterial amounts that related to prior periods.
Fair Value Measurements	Fair Value Measurements The carrying amounts reflected in the Consolidated Balance Sheets for cash and cash equivalents, accounts receivable, accounts payable, and accrued expenses approximate fair value due to their short-term maturities. See Note 3 – Marketable Investments for the fair value of the Company’s marketable investments.
Revenue Recognition	In May, 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers, which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The new standard will be effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. For Forrester, the standard will be effective in the first quarter of 2017. The two permitted transition methods under the new standard are the full retrospective method, in which case the standard would be applied to each prior reporting period presented, or the modified retrospective method, in which case the cumulative effect of applying the standard would be recognized at the date of initial application. The Company has not yet selected a transition method. The Company is currently evaluating the potential changes from this ASU to its future financial reporting and disclosures.

Disclosure of accounting policy for revenue recognition. If the entity has different policies for different types of revenue transactions, the policy for each material type of transaction is generally disclosed. If a sales transaction has multiple element arrangements (for example, delivery of multiple products, services or the rights to use assets) the disclosure may indicate the accounting policy for each unit of accounting as well as how units of accounting are determined and valued. The disclosure may encompass important judgment as to appropriateness of principles related to recognition of revenue. The disclosure also may indicate the entity's treatment of any unearned or deferred revenue that arises from the transaction.

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