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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY SECURITIES FUND
Prospectus Date rr_ProspectusDate Sep. 29, 2018
07.31 Fidelity OTC Portfolio K PRO-04 | Fidelity® OTC Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Fidelity® OTC Portfolio</b>/K</p>
Objective [Heading] rr_ObjectiveHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks capital appreciation.
Expense [Heading] rr_ExpenseHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>
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 <p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>
(fees paid directly from your investment) rr_ShareholderFeeOther none
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Portfolio Turnover</b></font>
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 38% of the average value of its portfolio.

Portfolio Turnover, Rate rr_PortfolioTurnoverRate 38.00%
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 <p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>
Strategy [Heading] rr_StrategyHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
  • Normally investing primarily in common stocks.
  • Normally investing at least 80% of assets in securities principally traded on NASDAQ® or an over-the-counter (OTC) market, which has more small and medium-sized companies than other markets.
  • Investing more than 25% of total assets in the technology sector.
  • 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.
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration <font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the technology sector.</font>
Risk [Heading] rr_RiskHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>
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.
  • Technology Industry Concentration. The technology industries can be significantly affected by obsolescence of existing technology, short product cycles, falling prices and profits, competition from new market entrants, and general economic conditions.
  • 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.

In addition, the fund is classified as non-diversified under the Investment Company Act of 1940, which means that it has the ability to invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified fund.

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 <font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>
Risk Nondiversified Status [Text] rr_RiskNondiversifiedStatus <font style="FONT-FAMILY: Times New Roman" size="3">In addition, the fund is classified as non-diversified under the Investment Company Act of 1940, which means that it has the ability to invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified fund.</font>
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution <font style="FONT-FAMILY: Times New Roman" size="3">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.</font>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>
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 more recent performance information.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <font style="FONT-FAMILY: Times New Roman" size="3">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.</font>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <font style="FONT-FAMILY: Times New Roman" size="3">www.401k.com and log in </font>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>
Bar Chart [Heading] rr_BarChartHeading <p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the periods shown in the chart:ReturnsQuarter ended
Highest Quarter Return24.55%June 30, 2009
Lowest Quarter Return-13.85%September 30, 2011
Year-to-Date Return12.34%June 30, 2018
Performance Table Heading rr_PerformanceTableHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2017
07.31 Fidelity OTC Portfolio K PRO-04 | Fidelity® OTC Portfolio | Fidelity OTC Portfolio-Class K  
Risk/Return: rr_RiskReturnAbstract  
Management fee (fluctuates based on the fund's performance relative to a securities market index) rr_ManagementFeesOverAssets 0.72%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.06%
Total annual operating expenses rr_ExpensesOverAssets 0.78%
1 year rr_ExpenseExampleYear01 $ 80
3 years rr_ExpenseExampleYear03 249
5 years rr_ExpenseExampleYear05 433
10 years rr_ExpenseExampleYear10 $ 966
2009 rr_AnnualReturn2009 62.60%
2010 rr_AnnualReturn2010 20.34%
2011 rr_AnnualReturn2011 (0.29%)
2012 rr_AnnualReturn2012 11.45%
2013 rr_AnnualReturn2013 46.70%
2014 rr_AnnualReturn2014 16.62%
2015 rr_AnnualReturn2015 11.05%
2016 rr_AnnualReturn2016 3.23%
2017 rr_AnnualReturn2017 38.75%
Year to Date Return, Label rr_YearToDateReturnLabel Year-to-Date Return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Jun. 30, 2018
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 12.34%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 24.55%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter Return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (13.85%)
Inception Date rr_AverageAnnualReturnInceptionDate May 09, 2008
07.31 Fidelity OTC Portfolio K PRO-04 | Fidelity® OTC Portfolio | Return Before Taxes | Fidelity OTC Portfolio-Class K  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity® OTC Portfolio
Label rr_AverageAnnualReturnLabel <font style="FONT-FAMILY: Times New Roman" ><b>Class K</b></font>
Past 1 year rr_AverageAnnualReturnYear01 38.75%
Past 5 years rr_AverageAnnualReturnYear05 22.17%
Since Inception rr_AverageAnnualReturnSinceInception 13.84% [1]
07.31 Fidelity OTC Portfolio K PRO-04 | Fidelity® OTC Portfolio | Nasdaq Composite Index®(reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 29.64%
Past 5 years rr_AverageAnnualReturnYear05 19.40%
Since Inception rr_AverageAnnualReturnSinceInception 12.60% [1]
[1] From May 9, 2008