Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY MT VERNON STREET TRUST
11.30 Fidelity New Millennium Fund - PRO-07 | Fidelity® New Millennium Fund®
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Summary Fund:Fidelity® New Millennium 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 19% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	19.00%
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	Differs from the ratios of expenses to average net assets in the Financial Highlights section of the prospectus because of acquired fund fees and expenses.
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. Identifying early signs of long-term changes in the marketplace and focusing on those companies that may benefit from opportunities created by these changes by examining technological advances, product innovation, economic plans, demographics, social attitudes, and other factors, which can lead to investments in small and medium-sized companies. Investing in domestic and foreign issuers. Investing in either "growth" stocks or "value" stocks or both. 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. "Value" Investing. "Value" stocks can perform differently from the market as a whole and other types of stocks and can continue to be undervalued by the market for long periods of time. 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
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 Explanation after Tax Higher	rr_PerformanceTableExplanationAfterTaxHigher	Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.
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). Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.
11.30 Fidelity New Millennium Fund - PRO-07 | Fidelity® New Millennium Fund® | Fidelity New Millennium Fund-Default
Risk/Return:	rr_RiskReturnAbstract
Management fee (fluctuates based on the fund's performance relative to a securities market index)	rr_ManagementFeesOverAssets	0.32%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.17%
Acquired fund fees and expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.10%
Total annual operating expenses	rr_ExpensesOverAssets	0.59%	[1]
1 year	rr_ExpenseExampleYear01	$ 60
3 years	rr_ExpenseExampleYear03	189
5 years	rr_ExpenseExampleYear05	329
10 years	rr_ExpenseExampleYear10	$ 738
2012	rr_AnnualReturn2012	15.66%
2013	rr_AnnualReturn2013	37.19%
2014	rr_AnnualReturn2014	6.96%
2015	rr_AnnualReturn2015	(3.17%)
2016	rr_AnnualReturn2016	14.92%
2017	rr_AnnualReturn2017	20.06%
2018	rr_AnnualReturn2018	(7.01%)
2019	rr_AnnualReturn2019	26.16%
2020	rr_AnnualReturn2020	5.76%
2021	rr_AnnualReturn2021	23.91%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Highest Quarter Return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2020
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	19.60%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Lowest Quarter Return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2020
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(27.69%)
11.30 Fidelity New Millennium Fund - PRO-07 | Fidelity® New Millennium Fund® | Return Before Taxes | Fidelity New Millennium Fund-Default
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return Before Taxes
Past 1 year	rr_AverageAnnualReturnYear01	23.91%
Past 5 years	rr_AverageAnnualReturnYear05	13.04%
Past 10 years	rr_AverageAnnualReturnYear10	13.30%
11.30 Fidelity New Millennium Fund - PRO-07 | Fidelity® New Millennium Fund® | After Taxes on Distributions | Fidelity New Millennium Fund-Default
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions
Past 1 year	rr_AverageAnnualReturnYear01	21.27%
Past 5 years	rr_AverageAnnualReturnYear05	10.57%
Past 10 years	rr_AverageAnnualReturnYear10	11.08%
11.30 Fidelity New Millennium Fund - PRO-07 | Fidelity® New Millennium Fund® | After Taxes on Distributions and Sales | Fidelity New Millennium Fund-Default
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year	rr_AverageAnnualReturnYear01	15.37%
Past 5 years	rr_AverageAnnualReturnYear05	9.83%
Past 10 years	rr_AverageAnnualReturnYear10	10.44%
11.30 Fidelity New Millennium Fund - PRO-07 | Fidelity® New Millennium Fund® | SP001
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	S&P 500® Index
Past 1 year	rr_AverageAnnualReturnYear01	28.71%
Past 5 years	rr_AverageAnnualReturnYear05	18.47%
Past 10 years	rr_AverageAnnualReturnYear10	16.55%

[1] ^(a)Differs from the ratios of expenses to average net assets in the Financial Highlights section of the prospectus because of acquired fund fees and expenses.