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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY MT VERNON STREET TRUST
Prospectus Date rr_ProspectusDate Jan. 29, 2014
K | Fidelity Growth Strategies Fund
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Summary

Fund/Class:
Fidelity® Growth Strategies Fund/K
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)
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 87% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 87.00%
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:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
  • Normally investing primarily in common stocks.
  • Normally investing in companies Fidelity Management & Research Company (FMR) believes offer the potential for accelerated earnings or revenue growth (stocks of these companies are often called "growth" stocks).
  • Focusing investments in medium-sized companies, but may also invest substantially in larger or smaller companies.
  • 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. The value of securities of smaller issuers can be more volatile than that of larger issuers.
  • "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 is not an indication of future performance.

Visit www.401k.com and log in for updated return 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.401k.com and log in for updated return information
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture Past performance 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:
Returns
Quarter ended
Highest Quarter Return
17.48%
September 30, 2009
Lowest Quarter Return
-20.99%
September 30, 2011
Performance Table Heading rr_PerformanceTableHeading Average Annual Returns
For the periods ended
December 31, 2013
K | Fidelity Growth Strategies Fund | Class K
  
Risk/Return: rr_RiskReturnAbstract  
Redemption fee on shares held less than 90 days (as a % of amount redeemed) rr_RedemptionFeeOverRedemption 1.50%
Management fee (fluctuates based on the fund's performance relative to a securities market index) rr_ManagementFeesOverAssets 0.39%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.09%
Total annual operating expenses rr_ExpensesOverAssets 0.48%
1 year rr_ExpenseExampleYear01 $ 49
3 years rr_ExpenseExampleYear03 154
5 years rr_ExpenseExampleYear05 269
10 years rr_ExpenseExampleYear10 $ 604
2009 rr_AnnualReturn2009 40.15%
2010 rr_AnnualReturn2010 25.89%
2011 rr_AnnualReturn2011 (8.72%)
2012 rr_AnnualReturn2012 12.04%
2013 rr_AnnualReturn2013 38.18%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 17.48%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter Return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (20.99%)
Label rr_AverageAnnualReturnLabel Class K
Past 1 year rr_AverageAnnualReturnYear01 38.18%
Past 5 years rr_AverageAnnualReturnYear05 20.05%
Life of class rr_AverageAnnualReturnSinceInception 6.71% [1]
Inception Date rr_AverageAnnualReturnInceptionDate May 09, 2008
K | Fidelity Growth Strategies Fund | Russell Midcap® Growth Index (reflects no deduction for fees, expenses or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Russell Midcap® Growth Index
Past 1 year rr_AverageAnnualReturnYear01 35.74%
Past 5 years rr_AverageAnnualReturnYear05 23.37%
Life of class rr_AverageAnnualReturnSinceInception 9.11% [1]
Inception Date rr_AverageAnnualReturnInceptionDate May 09, 2008
[1] From May 9, 2008.