Risk/Return Detail Data - FidelityNewMillenniumFund-RetailPRO	Jan. 28, 2023 USD ($)
Risk/Return:
Registrant Name	Fidelity Mt. Vernon Street Trust
FidelityNewMillenniumFund-RetailPRO | Fidelity New Millennium Fund
Risk/Return:
Risk/Return [Heading]	Fund Summary Fund: Fidelity® New Millennium Fund®
Objective [Heading]	Investment Objective
Objective, Primary [Text Block]	Fidelity® New Millennium 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 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 12 % of the average value of its portfolio.
Portfolio Turnover, Rate	12.00%
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	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]	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. 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]	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. 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]	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 (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]	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.fidelity.com
Performance Past Does Not Indicate Future [Text]	Past performance (before and after taxes) 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	Dec. 31, 2020
Highest Quarterly Return	19.60%
Lowest Quarterly Return, Label	Lowest Quarter Return
Lowest Quarterly Return, Date	Mar. 31, 2020
Lowest Quarterly Return	(27.69%)
Performance Table Heading	Average Annual Returns
Performance Table Uses Highest Federal Rate	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	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	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	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.
FidelityNewMillenniumFund-RetailPRO | Fidelity New Millennium Fund | Fidelity New Millennium Fund
Risk/Return:
Management fee	0.38%	[1]
Distribution and/or Service (12b-1) fees	none
Other expenses	0.16%
Acquired fund fees and expenses	0.07%
Total annual operating expenses	0.61%	[2]
1 year	$ 62
3 years	195
5 years	340
10 years	$ 762
2013	37.19%
2014	6.96%
2015	(3.17%)
2016	14.92%
2017	20.06%
2018	(7.01%)
2019	26.16%
2020	5.76%
2021	23.91%
2022	(1.56%)
FidelityNewMillenniumFund-RetailPRO | Fidelity New Millennium Fund | Return Before Taxes | Fidelity New Millennium Fund
Risk/Return:
Label	Return Before Taxes
Past 1 year	(1.56%)
Past 5 years	8.64%
Past 10 years	11.49%
FidelityNewMillenniumFund-RetailPRO | Fidelity New Millennium Fund | After Taxes on Distributions | Fidelity New Millennium Fund
Risk/Return:
Label	Return After Taxes on Distributions
Past 1 year	(2.77%)
Past 5 years	6.51%
Past 10 years	9.35%
FidelityNewMillenniumFund-RetailPRO | Fidelity New Millennium Fund | After Taxes on Distributions and Sales | Fidelity New Millennium Fund
Risk/Return:
Label	Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year	(0.51%)
Past 5 years	6.40%
Past 10 years	8.92%
FidelityNewMillenniumFund-RetailPRO | Fidelity New Millennium Fund | SP001
Risk/Return:
Label	S&P 500® Index
Past 1 year	(18.11%)
Past 5 years	9.42%
Past 10 years	12.56%

[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 S&P 500® Index. Please see "Fund Services - Fund Management - Advisory Fee(s)" for additional information.

[2] B 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.