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

Alger Mid 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)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination Feb. 28, 2017
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 122.05% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 122.05%
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.


The Fund focuses on mid-sized companies that Fred Alger Management, Inc. believes demonstrate promising growth potential. Under normal circumstances, the Fund invests at least 80% of its net assets in equity securities of companies that, at the time of purchase of the securities, have total market capitalization within the range of companies included in the Russell Midcap Growth Index or the S&P MidCap 400 Index, as reported by the indexes as of the most recent quarter-end. Both indexes are designed to track the performance of medium-capitalization stocks. At December 31, 2015, the market capitalization of the companies in these indexes ranged from $661.4 million to $30.4 billion.


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:


•  Mid Cap Securities Risk – there may be greater risk in investing in medium-capitalization 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.


•  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. Class Z Shares were not offered prior to May 28, 2015. Performance shown is that of the Fund's Class A Shares. Performance in the bar chart does not reflect the effect of the sales charge imposed on purchases of Class A Shares of the Fund. If the bar chart reflected the applicable sales charges, returns would be less than those shown. 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 One Year or Less [Text] rr_PerformanceOneYearOrLess Class Z Shares were not offered prior to May 28, 2015.
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 A Shares as of December 31 (%)
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads Performance in the bar chart does not reflect the effect of the sales charge imposed on purchases of Class A Shares of the Fund. If the bar chart reflected the applicable sales charges, returns would be less than those shown.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Best Quarter:
Q2 2009 19.64%		   Worst Quarter:
Q4 2008 -32.40%		  
Bar Chart, Returns for Class Not Offered in Prospectus [Text] rr_BarChartReturnsForClassNotOfferedInProspectus Performance shown is that of the Fund's Class A Shares.
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 19.64%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (32.40%)
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 Z Prospectus | Alger Mid Cap Growth Fund | Russell Midcap Growth Index (reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (0.20%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.54%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 8.16%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 8.27%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 1996
Class Z Prospectus | Alger Mid Cap Growth Fund | Alger Mid Cap Growth Fund Class Z  
Risk/Return: rr_RiskReturnAbstract  
Shareholder Fees (fees paid directly from your investment) rr_MaximumAccountFeeOverAssets none
Management Fees rr_ManagementFeesOverAssets 0.76%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 1.00%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.76%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.71%) [1]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 1.05%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 107
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 485
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 888
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 2,014
Class Z Prospectus | Alger Mid Cap Growth Fund | Alger Mid Cap Growth Fund Class A  
Risk/Return: rr_RiskReturnAbstract  
Annual Return 2006 rr_AnnualReturn2006 9.72%
Annual Return 2007 rr_AnnualReturn2007 31.51%
Annual Return 2008 rr_AnnualReturn2008 (58.49%)
Annual Return 2009 rr_AnnualReturn2009 50.78%
Annual Return 2010 rr_AnnualReturn2010 18.73%
Annual Return 2011 rr_AnnualReturn2011 (8.25%)
Annual Return 2012 rr_AnnualReturn2012 16.09%
Annual Return 2013 rr_AnnualReturn2013 35.60%
Annual Return 2014 rr_AnnualReturn2014 7.62%
Annual Return 2015 rr_AnnualReturn2015 (1.77%)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (6.97%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 7.67%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 4.48%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.88%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 1996
Class Z Prospectus | Alger Mid Cap Growth Fund | Alger Mid Cap Growth Fund Class A | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (6.97%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 7.67%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 3.60%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 6.53%
Class Z Prospectus | Alger Mid Cap Growth Fund | Alger Mid Cap Growth Fund Class A | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (3.94%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 6.04%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 3.25%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 6.09%
[1] Fred Alger Management, Inc. has contractually agreed to reimburse Fund expenses (excluding interest, taxes, brokerage and extraordinary expenses) through February 28, 2017, to the extent necessary to limit the annual operating expenses of the Fund's Class Z Shares to 1.05% of the class's average net assets. This expense reimbursement cannot be terminated. Fred Alger Management, Inc. may recoup reimbursed expenses during the one-year term of the expense reimbursement contract if the expense ratio falls below the stated limitation at the time of the reimbursement.