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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY MT VERNON STREET TRUST
Prospectus Date rr_ProspectusDate Jan. 29, 2018
11.30 Fidelity Growth Company Fund - Retail PRO-03 | Fidelity® Growth Company Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Growth Company Fund/Fidelity® Growth Company Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks capital appreciation.
Expense [Heading] rr_ExpenseHeading Fee Table
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

Shareholder fees
(fees paid directly from your investment) rr_ShareholderFeeOther none
Operating Expenses Caption [Text] rr_OperatingExpensesCaption

Annual Operating Expenses

(expenses that you pay each year as a % of the value of your investment)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
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. During the most recent fiscal year, the fund's portfolio turnover rate was 15% of the average value of its portfolio.

Portfolio Turnover, Rate rr_PortfolioTurnoverRate 15.00%
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

Sell All Shares
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
  • Normally investing primarily in common stocks.
  • Investing in companies that Fidelity Management & Research Company (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 Principal Investment Risks
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 You could lose money by investing in the fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution 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.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time. The index description appears in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

Visit www.fidelity.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.fidelity.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture Past performance (before and after taxes) is not an indication of future performance.
Bar Chart [Heading] rr_BarChartHeading

Year-by-Year Returns

Calendar Years
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the periods shown in the chart:ReturnsQuarter ended
Highest Quarter Return21.16%March 31, 2012
Lowest Quarter Return-23.46%December 31, 2008
Performance Table Heading rr_PerformanceTableHeading Average Annual Returns
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan).
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan).

For the periods ended December 31, 2017
11.30 Fidelity Growth Company Fund - Retail PRO-03 | Fidelity® Growth Company Fund | Fidelity Growth Company Fund-Retail Class  
Risk/Return: rr_RiskReturnAbstract  
Management fee (fluctuates based on the fund's performance relative to a securities market index) rr_ManagementFeesOverAssets 0.69%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.16%
Total annual operating expenses rr_ExpensesOverAssets 0.85%
1 year rr_ExpenseExampleYear01 $ 87
3 years rr_ExpenseExampleYear03 271
5 years rr_ExpenseExampleYear05 471
10 years rr_ExpenseExampleYear10 $ 1,049
2008 rr_AnnualReturn2008 (40.90%)
2009 rr_AnnualReturn2009 41.15%
2010 rr_AnnualReturn2010 20.55%
2011 rr_AnnualReturn2011 0.67%
2012 rr_AnnualReturn2012 18.52%
2013 rr_AnnualReturn2013 37.61%
2014 rr_AnnualReturn2014 14.44%
2015 rr_AnnualReturn2015 7.83%
2016 rr_AnnualReturn2016 6.01%
2017 rr_AnnualReturn2017 36.76%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2012
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 21.16%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter Return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (23.46%)
11.30 Fidelity Growth Company Fund - Retail PRO-03 | Fidelity® Growth Company Fund | Return Before Taxes | Fidelity Growth Company Fund-Retail Class  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity® Growth Company Fund
Label rr_AverageAnnualReturnLabel Return Before Taxes
Past 1 year rr_AverageAnnualReturnYear01 36.76%
Past 5 years rr_AverageAnnualReturnYear05 19.74%
Past 10 years rr_AverageAnnualReturnYear10 11.44%
11.30 Fidelity Growth Company Fund - Retail PRO-03 | Fidelity® Growth Company Fund | After Taxes on Distributions | Fidelity Growth Company Fund-Retail Class  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity® Growth Company Fund
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions
Past 1 year rr_AverageAnnualReturnYear01 35.30%
Past 5 years rr_AverageAnnualReturnYear05 18.34%
Past 10 years rr_AverageAnnualReturnYear10 10.68%
11.30 Fidelity Growth Company Fund - Retail PRO-03 | Fidelity® Growth Company Fund | After Taxes on Distributions and Sales | Fidelity Growth Company Fund-Retail Class  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity® Growth Company Fund
Label rr_AverageAnnualReturnLabel Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year rr_AverageAnnualReturnYear01 22.02%
Past 5 years rr_AverageAnnualReturnYear05 15.80%
Past 10 years rr_AverageAnnualReturnYear10 9.35%
11.30 Fidelity Growth Company Fund - Retail PRO-03 | Fidelity® Growth Company Fund | Russell 3000® Growth Index(reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 29.59%
Past 5 years rr_AverageAnnualReturnYear05 17.16%
Past 10 years rr_AverageAnnualReturnYear10 9.93%