Income Taxes	3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Note 6 — 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 benefit for the three months ended March 31, 2018 was $0.7 million resulting in an effective tax rate of 28.7% for the period. Income tax benefit for the three months ended March 31, 2017 was $0.1 million resulting in an effective tax rate of (3.0)% for the period. The increase in the effective tax rate during the three months ended March 31, 2018 compared to the prior year period was primarily due to the recognition of a $1.3 million benefit from the settlement of a tax audit in the first quarter of 2017. 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. We calculated our best estimate of the impact of the Act in our prior year end income tax provision in accordance with our 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. As the Company completes its analysis of the Act, and collects and prepares necessary data, and interprets any additional guidance issued by the U.S. Treasury Department, the IRS, and other standard-setting bodies, the Company may make adjustments to the provisional amounts. No adjustments to the provisional amounts were recognized during the three months ended March 31, 2018.

Note 6 — 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 benefit for the three months ended March 31, 2018 was $0.7 million resulting in an effective tax rate of 28.7% for the period. Income tax benefit for the three months ended March 31, 2017 was $0.1 million resulting in an effective tax rate of (3.0)% for the period. The increase in the effective tax rate during the three months ended March 31, 2018 compared to the prior year period was primarily due to the recognition of a $1.3 million benefit from the settlement of a tax audit in the first quarter of 2017. 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. We calculated our best estimate of the impact of the Act in our prior year end income tax provision in accordance with our 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. As the Company completes its analysis of the Act, and collects and prepares necessary data, and interprets any additional guidance issued by the U.S. Treasury Department, the IRS, and other standard-setting bodies, the Company may make adjustments to the provisional amounts. No adjustments to the provisional amounts were recognized during the three months ended March 31, 2018.