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

Alger Small 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 Feb. 28, 2019
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 44.56% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 44.56%
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 regardless of 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.


The Fund normally invests at least 80% of its net assets, plus any borrowings for investment purposes, in equity securities of companies that, at the time of purchase of the securities, have total market capitalization between (1) the higher of (a) $5 billion or (b) the company in either the Russell 2000 Growth Index or the MSCI USA Small Cap Index (each, an "Index" and together, the "Indexes") with the highest capitalization, and (2) the company in either Index with the lowest capitalization, at any time during the most recent 12-month period as reported by either Index. Both Indexes are broad-based indexes of small capitalization stocks. At December 31, 2017, the market capitalization of the companies in the Indexes ranged from $6.4 million to $13.2 billion. Equity securities include common or preferred stocks that are listed on U.S. or foreign exchanges. In addition, under normal market conditions, the Fund invests at least 25% of its total assets in technology companies focused in the fields of medicine and information. From time to time, the Fund may have 25% or more of its total assets invested in any one of these industries. Fred Alger Management, Inc. believes that there is very little in the field of medicine that does not use some type of medical technology and that has not been affected by new technology. Advances in technology increasingly pervade health care companies through changes in medical and surgical procedures, drugs, medical devices, and new support systems.


As a focus fund, the Fund intends to invest a substantial portion of its assets in a smaller number of issuers. The Fund will concentrate its portfolio in a few 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.


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 the purchased securities.


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 Cap Securities Risk – There may be greater risk in investing in smaller, less seasoned companies rather than larger, more established companies due to such factors as inexperienced management and limited product lines or financial resources. It may also be difficult or impossible to liquidate a security position at a time and price acceptable to the Fund because of 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.


Concentration Risk – Since the Fund concentrates its investments in technology companies, the value of the Fund's shares may be more volatile than mutual funds that do not similarly concentrate their investments. Furthermore because the field of medical technology is subject to substantial government regulation, changes in applicable regulations could adversely affect companies in those industries. In addition, the comparative rapidity of product development and technological advancement in those industries may be reflected in greater volatility of the stocks of companies operating in those areas. In addition, companies focused in the field of information technology can be significantly affected by intense competition, aggressive pricing, technological innovations, product obsolescence, patent considerations, product compatibility and consumer preferences.


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 audit and legal standards.
Risk Lose Money [Text] rr_RiskLoseMoney The Fund's share price may go down, which means you could lose money.
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 March 1, 2017. Historical performance prior to March 1, 2017 is that of the Fund's Class Z Shares. Prior to August 7, 2015, the Fund followed different investment strategies under the name "Alger Growth Opportunities Fund" and prior to February 12, 2015 was managed by a different portfolio manager. 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 March 1, 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 Z Shares as of December 31 (%)
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Best Quarter:
Q1 2012 14.33%		   Worst Quarter:
Q3 2011 -22.98%		  
Bar Chart, Returns for Class Not Offered in Prospectus [Text] rr_BarChartReturnsForClassNotOfferedInProspectus The returns shown are for Class Z Shares, which would have 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.
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2012
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 14.33%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Sep. 30, 2011
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (22.98%)
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 when 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 when 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, 2017
Class Y Prospectus | Alger Small Cap Focus Fund | Russell 2000 Growth Index (reflects no deductions for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 22.17%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 15.21%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 12.19%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 29, 2010
Class Y Prospectus | Alger Small Cap Focus Fund | Class Y  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management Fees rr_ManagementFeesOverAssets 0.75%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.76%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.51%
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.61%) [1]
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 0.90%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 92
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 417
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 766
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,749
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 92
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 417
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 766
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,749
Class Y Prospectus | Alger Small Cap Focus Fund | Alger Small Cap Focus Fund Class Z  
Risk/Return: rr_RiskReturnAbstract  
Annual Return 2011 rr_AnnualReturn2011 (4.79%)
Annual Return 2012 rr_AnnualReturn2012 14.22%
Annual Return 2013 rr_AnnualReturn2013 38.63%
Annual Return 2014 rr_AnnualReturn2014 2.16%
Annual Return 2015 rr_AnnualReturn2015 2.28%
Annual Return 2016 rr_AnnualReturn2016 8.48%
Annual Return 2017 rr_AnnualReturn2017 29.27%
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 29.27% [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 15.23% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 11.91% [2]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 29, 2010
Class Y Prospectus | Alger Small Cap Focus Fund | Alger Small Cap Focus Fund Class Z | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 28.98% [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 13.70% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 10.36% [2]
Class Y Prospectus | Alger Small Cap Focus Fund | Alger Small Cap Focus Fund Class Z | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 16.80% [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.75% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 9.26% [2]
[1] Fred Alger Management, Inc. has contractually agreed to waive fees or to reimburse Fund expenses (excluding acquired fund fees and expenses, dividend expense on short sales, borrowing costs, interest, taxes, brokerage and extraordinary expenses) through February 28, 2019 to the extent necessary to limit the annual operating expenses of Class Y Shares of the Fund to .90% of the class's average net assets. This expense reimbursement may only be amended or terminated prior to its expiration date by agreement between Fred Alger Management, Inc. and the Fund's Board of Trustees, and will terminate automatically in the event of termination of the Investment Advisory Agreement. 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 Z Shares, which would have 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.