Income Taxes	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Note 7 — Income Taxes Forrester provides for income taxes on an interim basis according to management’s estimate of the effective tax rate expected to be applicable for the full fiscal year. Certain items such as changes in tax rates, tax benefits or expense related to settlements of share-based payment awards, and foreign currency gains or losses are treated as discrete items and are recorded in the period in which they arise. Income tax expense for the nine months ended September 30, 2018 was $4.1 million resulting in an effective tax rate of 29.0% for the period. Income tax expense for the nine months ended September 30, 2017 was $6.3 million resulting in an effective tax rate of 32.6% for the period. Income tax expense decreased by $2.2 million during the nine months ended September 30, 2018 compared to the prior year period primarily due to a decrease in income before taxes for the 2018 period and a reduction in the federal tax rate due to the Tax Cuts and Jobs Act. These decreases were partially offset by a $1.3 million tax benefit recognized in the 2017 period from the settlement of a tax audit that did not recur in 2018. For the full year 2018, the Company anticipates that its effective tax rate will be approximately 31%. On December 22, 2017, the Tax Cuts and Jobs Act (the “Act”) was signed into law making significant changes to the Internal Revenue Code. The Company calculated its best estimate of the impact of the Act in its prior year end income tax provision in accordance with its understanding of the Act and guidance available at that date. On December 22, 2017, Staff Accounting Bulletin No. 118 ("SAB 118") was issued to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Act. SAB 118 provides a measurement period that should not extend beyond one year from the enactment date of the Act for companies to complete the accounting for the income tax effects of the Act. Any subsequent adjustment to these provisional amounts will be recorded to current tax expense in the quarter of 2018 when the analysis is complete. An immaterial adjustment to the provisional amount was recognized during the nine months ended September 30, 2018. In July 2015, the U.S. Tax Court issued an opinion in Altera Corp. v. Commissioner related to the treatment of stock-based compensation expense in an intercompany cost-sharing arrangement. The opinion invalidated part of a treasury regulation requiring stock-based compensation to be included in any qualified intercompany cost-sharing arrangement. The Company previously recorded a tax benefit based on the opinion in the case. Currently the U.S. Court of Appeals for the Ninth Circuit is reviewing the case and a final decision is yet to be issued. The Company will continue to monitor ongoing developments and potential impacts to its consolidated financial statements.

Note 7 — Income Taxes

Forrester provides for income taxes on an interim basis according to management’s estimate of the effective tax rate expected to be applicable for the full fiscal year. Certain items such as changes in tax rates, tax benefits or expense related to settlements of share-based payment awards, and foreign currency gains or losses are treated as discrete items and are recorded in the period in which they arise.

Income tax expense for the nine months ended September 30, 2018 was $4.1 million resulting in an effective tax rate of 29.0% for the period. Income tax expense for the nine months ended September 30, 2017 was $6.3 million resulting in an effective tax rate of 32.6% for the period. Income tax expense decreased by $2.2 million during the nine months ended September 30, 2018 compared to the prior year period primarily due to a decrease in income before taxes for the 2018 period and a reduction in the federal tax rate due to the Tax Cuts and Jobs Act. These decreases were partially offset by a $1.3 million tax benefit recognized in the 2017 period from the settlement of a tax audit that did not recur in 2018. For the full year 2018, the Company anticipates that its effective tax rate will be approximately 31%.

On December 22, 2017, the Tax Cuts and Jobs Act (the “Act”) was signed into law making significant changes to the Internal Revenue Code. The Company calculated its best estimate of the impact of the Act in its prior year end income tax provision in accordance with its understanding of the Act and guidance available at that date. On December 22, 2017, Staff Accounting Bulletin No. 118 ("SAB 118") was issued to address the application of U.S. GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Act. SAB 118 provides a measurement period that should not extend beyond one year from the enactment date of the Act for companies to complete the accounting for the income tax effects of the Act. Any subsequent adjustment to these provisional amounts will be recorded to current tax expense in the quarter of 2018 when the analysis is complete. An immaterial adjustment to the provisional amount was recognized during the nine months ended September 30, 2018.

In July 2015, the U.S. Tax Court issued an opinion in Altera Corp. v. Commissioner related to the treatment of stock-based compensation expense in an intercompany cost-sharing arrangement. The opinion invalidated part of a treasury regulation requiring stock-based compensation to be included in any qualified intercompany cost-sharing arrangement. The Company previously recorded a tax benefit based on the opinion in the case. Currently the U.S. Court of Appeals for the Ninth Circuit is reviewing the case and a final decision is yet to be issued. The Company will continue to monitor ongoing developments and potential impacts to its consolidated financial statements.