Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	ALGER FUNDS
Prospectus Date	rr_ProspectusDate	Mar. 01, 2020
Class Z Shares | Alger SmallCap Growth Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	ALGER SMALL CAP GROWTH FUND
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Alger Small 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. Investors may also incur brokerage commissions and other charges when buying or selling shares of the Fund, which are not reflected in the Example that follows.
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	February 28, 2021
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 17.09% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	17.09%
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-year examples are based on net operating expenses, which reflect the contractual expense limitation agreed to by Fred Alger Management, LLC. 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, LLC 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, rapidly growing demand or market dominance. Positive Lifecycle Change companies are, for example, companies benefitting from new regulations, a new product innovation or new management. The Fund focuses on small, fast-growing companies that Fred Alger Management, LLC believes offer innovative products, services or technologies to a rapidly-expanding marketplace. 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, 2019, the market capitalization of the companies in the Indexes ranged from $13 million to $27.34 billion. Because of the Fund’s long-term approach to investing, it could have a significant portion of its assets invested in securities of issuers that have appreciated beyond the market capitalization thresholds noted. Equity securities include common or preferred stocks that are listed on U.S. or foreign exchanges. The Fund may invest a significant portion of its assets in securities of companies conducting business within a single sector, including the healthcare and information technology 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. The Fund can invest in foreign securities, including emerging market 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 companies with small 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 because of the potentially less frequent trading of stocks of smaller market capitalization. Sector Risk – The Fund may have a significant portion of its assets invested in securities of companies conducting business within a single 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. Technology Companies Risk – The Fund may have a significant portion of its assets invested in securities of technology companies. Many technology companies’ securities have historically been more volatile than other securities, especially over the short term. Technology companies may also face increased competition, government regulation, and risk of obsolescence due to progress in technological developments. Healthcare Companies Risk – The Fund may have a significant portion of its assets invested in securities of healthcare companies. At times, the performance of healthcare companies will lag the performance of other industries or the broader market as a whole, and the performance of such companies may be more volatile. Healthcare companies may also be significantly affected by intense competition, aggressive pricing, government regulation, technological innovations, product obsolescence, patent considerations, product compatibility and consumer preferences. 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. The securities of issuers located in emerging markets can be more volatile than those of issuers in more mature economies. Emerging Markets Risk – The Fund may invest in issuers located in emerging countries (such as Brazil, Russia, India and China), and therefore may be exposed to the economies, industries, securities and currency markets of such countries, which may be adversely affected by protectionist trade policies, a slow U.S. economy, political and social instability, regional and global conflicts, terrorism and war, including actions that are contrary to the interests of the U.S.
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. 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 Z Shares as of December 31 (%)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter: Q1 2019 19.16%               Worst Quarter: Q3 2011 -22.89%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Return as of December 31, 2019
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	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	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.
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 Narrative	rr_PerformanceTableNarrativeTextBlock	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.
Class Z Shares | Alger SmallCap Growth Fund | Class Z
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Advisory Fees	rr_ManagementFeesOverAssets	0.81%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.31%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.12%
Fee Waiver and/or Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.13%)	[1]
Total Annual Fund Operating Expenses	rr_NetExpensesOverAssets	0.99%	[2]
1 Year	rr_ExpenseExampleYear01	$ 101
3 Years	rr_ExpenseExampleYear03	343
5 Years	rr_ExpenseExampleYear05	604
10 Years	rr_ExpenseExampleYear10	1,352
1 Year	rr_ExpenseExampleNoRedemptionYear01	101
3 Years	rr_ExpenseExampleNoRedemptionYear03	343
5 Years	rr_ExpenseExampleNoRedemptionYear05	604
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,352
2011	rr_AnnualReturn2011	(2.98%)
2012	rr_AnnualReturn2012	12.45%
2013	rr_AnnualReturn2013	34.19%
2014	rr_AnnualReturn2014	0.15%
2015	rr_AnnualReturn2015	(3.23%)
2016	rr_AnnualReturn2016	5.94%
2017	rr_AnnualReturn2017	28.64%
2018	rr_AnnualReturn2018	2.32%
2019	rr_AnnualReturn2019	28.97%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2019
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	19.16%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(22.89%)
1 Year	rr_AverageAnnualReturnYear01	28.97%
5 Years	rr_AverageAnnualReturnYear05	11.72%
Since Inception	rr_AverageAnnualReturnSinceInception	10.88%
Inception Date	rr_AverageAnnualReturnInceptionDate	Dec. 29, 2010
Class Z Shares | Alger SmallCap Growth Fund | Return After Taxes on Distributions | Class Z
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	27.33%
5 Years	rr_AverageAnnualReturnYear05	9.74%
Since Inception	rr_AverageAnnualReturnSinceInception	9.02%
Inception Date	rr_AverageAnnualReturnInceptionDate	Dec. 29, 2010
Class Z Shares | Alger SmallCap Growth Fund | Return After Taxes on Distributions and Sale of Shares | Class Z
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	18.31%
5 Years	rr_AverageAnnualReturnYear05	8.86%
Since Inception	rr_AverageAnnualReturnSinceInception	8.40%
Inception Date	rr_AverageAnnualReturnInceptionDate	Dec. 29, 2010
Class Z Shares | Alger SmallCap Growth Fund | Russell 2000 Growth Index (reflects no deductions for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	28.48%
5 Years	rr_AverageAnnualReturnYear05	9.34%
Since Inception	rr_AverageAnnualReturnSinceInception	11.23%

[1] Fred Alger Management, LLC 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, 2021 to the extent necessary to limit the total annual fund operating expenses of the Class Z Shares of the Fund to .99% of the class's average daily net assets. This expense reimbursement may only be amended or terminated prior to its expiration date by agreement between Fred Alger Management, LLC and the Fund's Board of Trustees, and will terminate automatically in the event of termination of the Investment Advisory Agreement.

[2] After Fee Waiver and/or Expense Reimbursement