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Class Y Prospectus | Alger SMid Cap Focus Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Alger SMid Cap Focus Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

Alger SMid Cap Focus Fund seeks long-term capital appreciation.
Expense [Heading] rr_ExpenseHeading Fund Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if 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 Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination Aug. 29, 2018
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 fund operating expenses or in the example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 164.36% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 164.36%
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated, that your investment has a 5% return each year and that the Fund's operating expenses remain the same. The one-year example and the first year of the three-, five- and ten-years examples are based on net operating expenses, which reflect the contractual expense limitation agreed to by Fred Alger Management, Inc.
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs may be higher or lower, based on these assumptions you would pay the following expenses whether or not you redeemed your shares at the end of each period:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategy
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

Fred Alger Management, Inc. believes companies undergoing Positive Dynamic Change offer the best investment opportunities. Positive Dynamic Change refers to companies realizing High Unit Volume Growth or companies undergoing Positive Lifecycle Change. High Unit Volume Growth companies are traditional growth companies experiencing, for example, significantly growing demand or market dominance. Positive Lifecycle Change companies are, for example, companies benefitting from regulatory change, a new product introduction or management change.


Weatherbie Capital, LLC, an affiliate of Fred Alger Management, Inc., invests in smaller cap U. S. growth companies that Weatherbie believes have enduring earnings, reasonable valuations and a distinct competitive advantage. Weatherbie Capital, LLC invests in Foundation growth stocks and Opportunity growth stocks. Foundation growth stocks are companies led by experienced management teams, with innovative business models and the potential for high sales and earnings growth. Opportunity growth stocks are companies whose earnings may be temporarily depressed, but Weatherbie believes change is underway that can reaccelerate earnings.


Under normal circumstances, the Fund invests at least 80% of its net assets, plus any borrowings for investment purposes, in equity securities of smallcap and midcap companies. Equity securities include common or preferred stocks that are listed on U.S. or foreign exchanges. Smallcap or midcap companies are companies that, at the time of purchase, have total market capitalization within the range of companies included in the Russell 2500 Growth Index or the Russell Midcap Growth Index, respectively, as reported by the indexes as of the most recent quarter-end. At June 30, 2017, the market capitalization of the companies in these indexes ranged from $90.4 million to $30.6 billion.


As a focus fund, the Fund intends to invest a substantial portion of its assets in a smaller number of issuers, and may focus its holdings in fewer business sectors or industries. Generally the Fund will own approximately 50 holdings. Fund holdings may occasionally exceed this number for a variety of reasons.


The Fund's portfolio manager(s) may sell a stock when it reaches a target price, it fails to perform as expected, or other opportunities appear more attractive. As a result, the Fund may engage in active trading of portfolio securities.


The Fund can leverage, that is, borrow money to buy additional securities. By borrowing money, the Fund has the potential to increase its returns if the increase in the value of the securities purchased exceeds the cost of borrowing, including interest paid on the money borrowed. However, leverage also has the potential to magnify any decrease in the value of borrowed securities.


The Fund can also invest in privately placed securities, which are securities acquired in non-public offerings for which there is no readily available market.


The Fund can invest in foreign securities.
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock

An investment in the Fund involves risks. The Fund's share price may go down, which means you could lose money. 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.


Investment Risk – An investment in the Fund is subject to investment risk, including the possible loss of the entire principal amount that you invest.


Market Risk – Your investment in Fund shares represents an indirect investment in the securities owned by the Fund. The value of these securities, like other investments, may move up or down, sometimes rapidly and unpredictably. Your Fund shares at any point in time may be worth less than what you invested, even after taking into account the reinvestment of Fund dividends and distributions.


Equity Securities Risk – As with any fund that invests in stocks, your investment will fluctuate in value, and the loss of your investment is a risk of investing. The Fund's price per share will fluctuate due to changes in the market prices of its investments. Also, the Fund's investments may not grow as fast as the rate of inflation and stocks tend to be more volatile than some other investments you could make, such as bonds.


Growth Stocks Risk – Prices of growth stocks tend to be higher in relation to their companies' earnings and may be more sensitive to market, political and economic developments than other stocks, making their prices more volatile. An investment in the Fund may be better suited to investors who seek long-term capital growth and can tolerate fluctuations in their investment's value.


Small and Mid Cap Securities Risk – there may be greater risk in investing in companies with small or medium market capitalizations rather than larger, more established issuers owing to such factors as more limited product lines or financial resources or lack of management depth. It may also be difficult or impossible to liquidate a security position at a time and price acceptable to the Fund owing to the potentially less frequent trading of stocks of smaller market capitalization.


Less Diversified Portfolio Risk – the Fund may have a more concentrated portfolio than other funds, so it may be more vulnerable to changes in the market value of a single issuer and may be more susceptible to risks associated with a single economic, political or regulatory occurrence than a fund that has a more diversified portfolio. The Fund may have substantial holdings within a particular sector, and companies in similar industries may be similarly affected by particular economic or market events.


Leverage Risk – the cost of borrowing money to leverage may exceed the returns for the securities purchased or the securities purchased may actually go down in value; thus the Fund's net asset value could decrease more quickly than if it had not borrowed.


Foreign Securities Risk – the Fund's performance will be influenced by political, social and economic factors affecting investments in foreign companies. Special risks associated with investments in foreign companies include exposure to currency fluctuations, less liquidity, less developed or less efficient trading markets, lack of comprehensive company information, political instability and differing auditing and legal standards.


Privately Placed Securities Risk – a private placement is an offering of a company's securities that is not registered with the SEC and is not offered to the public. Less information may be available about companies that make private placements than about publicly offered companies and such companies may not be subject to the disclosure and other investor protection requirements that would be applicable if their securities were publicly traded. The sale or transfer of privately placed securities may be limited or prohibited by contract or law and such investments are generally considered to be illiquid. Private placed securities are generally fair valued as they are not traded frequently. The Fund may be required to hold such positions for several years, if not longer, regardless of valuation, which may cause the Fund to be less liquid.


Portfolio Turnover Risk – the Fund may engage in active trading, which could produce higher transaction costs.
Risk Lose Money [Text] rr_RiskLoseMoney As with any fund that invests in stocks, your investment will fluctuate in value, and the loss of your investment is a risk of investing.
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 bar chart and the table beneath it provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year and by showing how the Fund's average annual returns for the indicated periods compare with those of an appropriate benchmark of market performance. Class Y Shares were not offered prior to August 30, 2017. Historical performance prior to August 30, 2017 is that of the Fund's Class I Shares. Performance prior to August 6, 2007, commencement of the Class I Shares' operations, is that of the Fund's Class A Shares. Prior to August 30, 2017, the Fund followed different investment strategies under the name "Alger SMid Cap Growth Fund" and prior to March 1, 2017 was managed by different portfolio managers. Accordingly, performance prior to those dates does not reflect the Fund's current investment strategies and investment personnel. The Fund's past performance (before and after taxes) is not necessarily an indication of how it will perform in the future. Updated performance information is available on the Fund's website www.alger.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The following bar chart and the table beneath it provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year and by showing how the Fund's average annual returns for the indicated periods compare with those of an appropriate benchmark of market performance.
Performance One Year or Less [Text] rr_PerformanceOneYearOrLess Class Y Shares were not offered prior to August 30, 2017.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.alger.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund's past performance (before and after taxes) is not necessarily an indication of how it will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Annual Total Return For Class I Shares* as of December 31 (%)
Bar Chart Footnotes [Text Block] rr_BarChartFootnotesTextBlock

*  The returns shown are for Class I Shares, which would have had substantially similar annual returns as Class Y Shares because the Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the Classes do not have the same expenses. Performance of the Fund's Class I Shares prior to August 6, 2007 reflects the performance of the Fund's Class A Shares, as adjusted with currently applicable sales charges and operating expenses, which differ from historical charges and expenses.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Best Quarter:
Q2 2009 18.02% 		  Worst Quarter:
Q4 2008 -28.32% 		 

The year-to-date total return of the Fund's Class I Shares as of June 30, 2017 was 19.88%.
Bar Chart, Returns for Class Not Offered in Prospectus [Text] rr_BarChartReturnsForClassNotOfferedInProspectus Historical performance prior to August 30, 2017 is that of the Fund's Class I Shares.
Year to Date Return, Label rr_YearToDateReturnLabel Year-to-date total return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Jun. 30, 2017
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 19.88%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 18.02%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (28.32%)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deductions for fees, expenses or taxes)
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate In the foregoing table, after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred The after-tax returns shown may not be relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher A "Return After Taxes on Distributions and Sale of Fund Shares" may sometimes be higher than the other two return figures; this happens where there is a capital loss on redemptions, giving rise to a tax benefit to the shareholder.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

In the foregoing table, after-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor's tax situation and may differ from those shown. The after-tax returns shown may not be relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. A "Return After Taxes on Distributions and Sale of Fund Shares" may sometimes be higher than the other two return figures; this happens where there is a capital loss on redemptions, giving rise to a tax benefit to the shareholder.
Caption rr_AverageAnnualReturnCaption Average Annual Total Return as of December 31, 2016
Class Y Prospectus | Alger SMid Cap Focus Fund | Russell 2500 Growth Index (reflects no deductions for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 9.73%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 13.88%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.32%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 8.25%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate May 08, 2002
Class Y Prospectus | Alger SMid Cap Focus Fund | Alger SMid Cap Focus Fund Class Y  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fees rr_ManagementFeesOverAssets 0.81%
Distribution and/or Shareholder Servicing (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.41%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.22%
Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.35% [1]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.87%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 89
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 353
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 637
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,446
Class Y Prospectus | Alger SMid Cap Focus Fund | Alger SMid Cap Focus Fund Class I  
Risk/Return: rr_RiskReturnAbstract  
Annual Return 2007 rr_AnnualReturn2007 26.54%
Annual Return 2008 rr_AnnualReturn2008 (48.56%)
Annual Return 2009 rr_AnnualReturn2009 44.33%
Annual Return 2010 rr_AnnualReturn2010 23.41%
Annual Return 2011 rr_AnnualReturn2011 (3.91%)
Annual Return 2012 rr_AnnualReturn2012 14.10%
Annual Return 2013 rr_AnnualReturn2013 32.14%
Annual Return 2014 rr_AnnualReturn2014 4.26%
Annual Return 2015 rr_AnnualReturn2015 (2.31%)
Annual Return 2016 rr_AnnualReturn2016 3.74%
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 3.74% [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 9.76% [2]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 5.91% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 5.07% [2]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Aug. 06, 2007
Class Y Prospectus | Alger SMid Cap Focus Fund | Alger SMid Cap Focus Fund Class I | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 1.12% [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 5.64% [2]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 3.87% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 2.93% [2]
Class Y Prospectus | Alger SMid Cap Focus Fund | Alger SMid Cap Focus Fund Class I | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 4.37% [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 7.55% [2]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 4.66% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 3.95% [2]
[1] Fred Alger Management, Inc. has contractually agreed to reimburse Fund expenses (excluding interest, taxes, brokerage, and extraordinary expenses) through August 29, 2018 to the extent necessary to limit the annual operating expenses of Class Y Shares of the Fund to .87% of the class's average net assets. This expense reimbursement cannot be terminated. Fred Alger Management, Inc. may, during the one-year term of the expense reimbursement contract, recoup any expenses waived or reimbursed pursuant to the expense reimbursement contract to the extent that such recoupment would not cause the expense ratio to exceed the lesser of the stated limitation in effect at the time of (i) the waiver or reimbursement and (ii) the recoupment.
[2] The returns shown are for Class I Shares, which would have had substantially similar annual returns as Class Y Shares because the Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the Classes do not have the same expenses. Performance of the Fund's Class I Shares prior to August 6, 2007 reflects the performance of the Fund's Class A Shares, as adjusted with currently applicable sales charges and operating expenses, which differ from historical charges and expenses.