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

Alger SMid Cap Growth 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 110.40% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 110.40%
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.


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. The Fund focuses on smallcap and midcap companies that Fred Alger Management, Inc. believes demonstrate promising growth potential. 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, 2015, the market capitalization of the companies in these indexes ranged from $18.7 million to $30.4 billion.


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.


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

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.


Investment Style 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.


The following risks may also apply:


•  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.


•  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.


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.
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. Performance prior to August 6, 2007, commencement of the Class I Share's operations, is that of the Fund's Class A Shares. Remember that 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 Remember that 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 deduction 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, 2015
Class I Prospectus | Alger SMid Cap Growth Fund | Russell 2500 Growth Index (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (0.19%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.43%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 8.49%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 8.69%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate May 08, 2002
Class I Prospectus | Alger SMid Cap Growth Fund | Alger SMid Cap Growth Fund Class I  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_MaximumAccountFeeOverAssets none
Management Fees rr_ManagementFeesOverAssets 0.81%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.25%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.31%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 133
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 415
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 718
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,579
Annual Return 2006 rr_AnnualReturn2006 17.85%
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%)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (2.31%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 8.09%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.26% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.97% [1]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Aug. 06, 2007
Class I Prospectus | Alger SMid Cap Growth Fund | Alger SMid Cap Growth Fund Class I | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (10.54%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 4.57%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 5.40% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 6.40% [1]
Class I Prospectus | Alger SMid Cap Growth Fund | Alger SMid Cap Growth Fund Class I | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 5.50%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 6.27%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 5.85% [1]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 6.54% [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.