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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY MT VERNON STREET TRUST
Prospectus Date rr_ProspectusDate Jan. 29, 2020
11.30 Fidelity Growth Company K6 Fund PRO-02 | Fidelity Growth Company K6 Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: bold 12pt Arial, Helvetica, sans-serif;">Fund:<br/>Fidelity® Growth Company K6 Fund</p>
Objective [Heading] rr_ObjectiveHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks capital appreciation.

Expense [Heading] rr_ExpenseHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text] rr_ShareholderFeesCaption <p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>
(fees paid directly from your investment) rr_ShareholderFeeOther none
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>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Portfolio Turnover</b></font>
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock

The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. For the period from June 13, 2019 to November 30, 2019, the fund's portfolio turnover rate was 16% of the average value of its portfolio.

Portfolio Turnover, Rate rr_PortfolioTurnoverRate 16.00%
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates <font style="FONT-FAMILY: Times New Roman" size="3">Based on estimated amounts for the current fiscal year.</font>
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption <p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>
Strategy [Heading] rr_StrategyHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
  • Normally investing primarily in common stocks.
  • Investing in companies that Fidelity Management & Research Company LLC (FMR) believes have above-average growth potential (stocks of these companies are often called "growth" stocks).
  • Investing in domestic and foreign issuers.
  • Using fundamental analysis of factors such as each issuer's financial condition and industry position, as well as market and economic conditions, to select investments.
Risk [Heading] rr_RiskHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
  • Stock Market Volatility. Stock markets are volatile and can decline significantly in response to adverse issuer, political, regulatory, market, or economic developments. Different parts of the market, including different market sectors, and different types of securities can react differently to these developments.
  • Foreign Exposure. Foreign markets can be more volatile than the U.S. market due to increased risks of adverse issuer, political, regulatory, market, or economic developments and can perform differently from the U.S. market.
  • Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole.
  • "Growth" Investing. "Growth" stocks can perform differently from the market as a whole and other types of stocks and can be more volatile than other types of stocks.

An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. You could lose money by investing in the fund.

Risk Lose Money [Text] rr_RiskLoseMoney <font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution <font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

Performance history will be available for the fund after the fund has been in operation for one calendar year.

Performance One Year or Less [Text] rr_PerformanceOneYearOrLess <font style="FONT-FAMILY: Times New Roman" size="3">Performance history will be available for the fund after the fund has been in operation for one calendar year.</font>
11.30 Fidelity Growth Company K6 Fund PRO-02 | Fidelity Growth Company K6 Fund | Fidelity Growth Company K6 Fund  
Risk/Return: rr_RiskReturnAbstract  
Management fee rr_ManagementFeesOverAssets 0.45%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets none [1]
Total annual operating expenses rr_ExpensesOverAssets 0.45%
1 year rr_ExpenseExampleYear01 $ 46
3 years rr_ExpenseExampleYear03 $ 144
[1] Based on estimated amounts for the current fiscal year.