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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName Fidelity Newbury Street Trust
Prospectus Date rr_ProspectusDate Dec. 30, 2019
10.31 Fidelity Tax-Exempt Money Market & Treasury Money Market Daily Money Combo PRO-09 | Fidelity® Treasury Money Market Fund  
Risk/Return: rr_RiskReturnAbstract  
Supplement to Prospectus [Text Block] rr_SupplementToProspectusTextBlock

Supplement to the
Fidelity® Tax-Exempt Money Market Fund and Fidelity® Treasury Money Market Fund
Daily Money Class
December 30, 2019
Prospectus
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination <font style="FONT-FAMILY: Times New Roman" size="3">February 28, 2021</font>
10.31 Fidelity Tax-Exempt Money Market & Treasury Money Market Daily Money Combo PRO-09 | Fidelity® Treasury Money Market Fund | Fidelity Treasury Money Market Fund-Daily Money Class  
Risk/Return: rr_RiskReturnAbstract  
Management fee rr_ManagementFeesOverAssets 0.25%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.22%
Total annual operating expenses rr_ExpensesOverAssets 0.72%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.02% [1]
Total annual operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.70% [2]
[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Daily Money Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.70% (the Expense Cap). If at any time during the current fiscal year expenses for Daily Money Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through February 28, 2021. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[2] In order to avoid a negative yield, FMR may reimburse expenses or waive fees of Daily Money Class of the fund and/or the fund’s distributor may waive all or a portion of the 12b-1 fees of Daily Money Class of the fund. Any such waivers or expense reimbursement would be voluntary and could be discontinued at any time. There is no guarantee that Daily Money Class or the fund will be able to avoid a negative yield.