Interim Consolidated Financial Statements (Policies)	6 Months Ended
Interim Consolidated Financial Statements (Policies)	Jun. 30, 2017
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, 2016. In the opinion of management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair statement of the financial position, results of operations, comprehensive income and cash flows as of the dates and for the periods presented have been included. The results of operations for the three and six months ended June 30, 2017 may not be indicative of the results for the year ending December 31, 2017, or any other period.
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.
Adoption of New Accounting Pronouncements	Adoption of New Accounting Pronouncements The Company adopted the guidance in Accounting Standards Update ("ASU") No. 2016-09, Compensation - Stock Compensation - Improvements to Employee Share-Based Payment Accounting, on January 1, 2017. Under this standard, entities are permitted to make an accounting policy election to either estimate forfeitures on share-based payment awards, as previously required, or to recognize forfeitures as they occur. The Company has elected to recognize forfeitures as they occur and the impact of that change in accounting policy has been recorded as a $0.2 million cumulative effect adjustment to increase retained earnings as of January 1, 2017. Additionally, ASU No. 2016-09 requires that all income tax effects related to settlements of share-based payment awards be reported in earnings as an increase or decrease to income tax expense. Previously, income tax effects at settlement of an award were reported as an increase (or decrease) to additional paid-in capital to the extent that those benefits were greater than (or less than) the income tax effects reported in earnings during the award's vesting period. The requirement to report those income tax effects in earnings has been applied on a prospective basis to settlements occurring on or after January 1, 2017, and the impact of applying this guidance was not material to the consolidated financial statements for the three and six months ended June 30, 2017. Application of this guidance may result in fluctuations in the Company’s effective tax rate depending on how many options are exercised, how many restricted stock units vest and the volatility of the Company’s stock price. ASU 2016-09 also requires that all income tax-related cash flows resulting from share-based payments be reported as operating activities in the statement of cash flows. Previously, income tax benefits at settlement of an award were reported as a reduction to operating cash flows and an increase to financing cash flows to the extent that those benefits exceeded the income tax benefits reported in earnings during the award's vesting period. In addition, the standard requires that cash paid by directly withholding shares for tax withholding purposes be classified as a financing activity in the statement of cash flows. For the six months ended June 30, 2017, the Company reflected $0.5 million of tax withholding in financing activities. The Company has elected to apply the changes in cash flow classification on a retrospective basis resulting in an increase in operating cash flows, with a corresponding decrease in financing cash flows, of $0.8 million for the six months ended June 30, 2016, as compared to the amounts previously reported. The Company elected to early adopt the guidance in ASU No. 2016-16, Accounting for Income Taxes: Intra-Entity Asset Transfers of Assets Other than Inventory, on January 1, 2017. The guidance in this standard eliminates for all intra-entity sales of assets other than inventory, the exception under existing standards that permits the tax effects of intra-entity asset transfers to be deferred until the transferred asset is sold to a third party or otherwise recovered through use. As a result, a reporting entity would recognize the tax expense from the sale of the asset in the seller’s tax jurisdiction when the transfer occurs and any deferred tax asset that arises in the buyer’s jurisdiction would also be recognized at the time of the transfer. As a result, the Company has recorded a $0.5 million cumulative effect adjustment to reduce retained earnings as of January 1, 2017.