Risk/Return Detail Data - FidelityGrowthCompanyFund-KPRO	Jan. 28, 2023 USD ($)
Risk/Return:
Registrant Name	Fidelity Mt. Vernon Street Trust
FidelityGrowthCompanyFund-KPRO | Fidelity Growth Company Fund
Risk/Return:
Risk/Return [Heading]	Fund Summary Fund /Class: Fidelity® Growth Company Fund /K
Objective [Heading]	Investment Objective
Objective, Primary [Text Block]	Fidelity® Growth Company Fund seeks capital appreciation.
Expense [Heading]	Fee Table
Expense Narrative [Text Block]	The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.
Shareholder Fees Caption [Text]	Shareholder fees
(fees paid directly from your investment)	none
Operating Expenses Caption [Text]	Annual Operating Expenses
Portfolio Turnover [Heading]	Portfolio Turnover
Portfolio Turnover [Text Block]	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 14 % of the average value of its portfolio.
Portfolio Turnover, Rate	14.00%
Expense Example Narrative [Text Block]	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 the 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]	Principal Investment Strategies
Strategy Narrative [Text Block]	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]	Principal Investment Risks
Risk Narrative [Text Block]	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]	You could lose money by investing in the fund.
Risk Not Insured Depository Institution [Text]	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]	Performance
Performance Narrative [Text Block]	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 more recent performance information.
Performance Information Illustrates Variability of Returns [Text]	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]	www.401k.com
Performance Past Does Not Indicate Future [Text]	Past performance is not an indication of future performance.
Bar Chart [Heading]	Year-by-Year Returns
Highest Quarterly Return, Label	Highest Quarter Return
Highest Quarterly Return, Date	Jun. 30, 2020
Highest Quarterly Return	40.62%
Lowest Quarterly Return, Label	Lowest Quarter Return
Lowest Quarterly Return, Date	Jun. 30, 2022
Lowest Quarterly Return	(25.00%)
Performance Table Heading	Average Annual Returns
FidelityGrowthCompanyFund-KPRO | Fidelity Growth Company Fund | Fidelity Growth Company Fund - Class K
Risk/Return:
Management fee	0.75%	[1]
Distribution and/or Service (12b-1) fees	none
Other expenses	0.04%
Total annual operating expenses	0.79%
1 year	$ 81
3 years	252
5 years	439
10 years	$ 978
2013	37.76%
2014	14.57%
2015	7.94%
2016	6.12%
2017	36.90%
2018	(4.46%)
2019	38.52%
2020	67.69%
2021	22.73%
2022	(33.74%)
FidelityGrowthCompanyFund-KPRO | Fidelity Growth Company Fund | Return Before Taxes | Fidelity Growth Company Fund - Class K
Risk/Return:
Label	Class K
Past 1 year	(33.74%)
Past 5 years	12.53%
Past 10 years	16.14%
FidelityGrowthCompanyFund-KPRO | Fidelity Growth Company Fund | RS007
Risk/Return:
Label	Russell 3000® Growth Index
Past 1 year	(28.97%)
Past 5 years	10.45%
Past 10 years	13.75%

[1] A The management fee is comprised of a basic fee of 0.53% adjusted up or down by a maximum of 0.20% based on the fund's performance relative to that of the Russell 3000® Growth Index. Please see "Fund Services - Fund Management - Advisory Fee(s)" for additional information.