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Class I 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)
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 157.39% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 157.39%
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.
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 ("Weatherbie"), an affiliate of Fred Alger Management, Inc. and the Fund's sub-adviser, invests in smaller cap U. S. growth companies that Weatherbie believes have enduring earnings, reasonable valuations and a distinct competitive advantage. Weatherbie 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 change is underway that Weatherbie believes 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 December 31, 2017, the market capitalization of the companies in these indexes ranged from $22.5 million to $36.7 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 may invest a significant portion of its assets in securities of companies conducting business within a single sector, including the information technology, consumer discretionary, health care, and industrials sectors.


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 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 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.


Sector Risk — The Fund may have a significant portion of its assets invested in securities of companies conducting business in a related group of industries within a sector. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund more vulnerable to unfavorable developments in that sector than a fund that has a more diversified portfolio. Generally, the more broadly the Fund invests, the more it spreads risk and potentially reduces the risks of loss and volatility.


The Fund may be more susceptible to particular risks that may affect companies in the information technology sector than if it were invested in a wider variety of companies in unrelated sectors. At times, the performance of such companies will lag the performance of other industries or the broader market as a whole. Certain technology related companies may face special risks that their products or services may not prove to be commercially successful. Technology related companies are also strongly affected by worldwide scientific or technological developments. As a result, their products may rapidly become obsolete. Such companies are also often subject to governmental regulation and may, therefore, be adversely affected by governmental policies. These factors may lead to limited earnings and/or failing profit margins. As a result, the value of technology related companies' securities may fall or fail to rise. In addition, many technology related companies have limited operating histories and prices of these companies' securities have historically been more volatile than other securities, especially over the short term.


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.


Portfolio Turnover (Active Trading) Risk – Because the Fund may engage in active trading of portfolio securities, it may incur increased transaction costs and brokerage commissions, both of which can lower the actual return on an investment. Active trading may also increase short-term gains and losses, which may affect the taxes a shareholder has to pay.
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. Performance prior to August 6, 2007, commencement of the Class I Share's 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 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

*  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%		  
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 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 I 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 24.46%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 15.47%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 9.62%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 9.70%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate May 08, 2002
Class I Prospectus | Alger SMid Cap Focus Fund | Alger SMid Cap Focus Fund Class I  
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 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.29%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.35%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 137
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 428
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 739
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,624
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 137
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 428
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 739
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,624
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%
Annual Return 2017 rr_AnnualReturn2017 37.90%
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 37.90% [1]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 14.00% [1]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.82% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 9.39% [1]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate May 08, 2002
Class I 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 31.67% [1]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 8.84% [1]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 4.31% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.51% [1]
Class I 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 21.92% [1]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 9.77% [1]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 4.81% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.41% [1]
[1] 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.