Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	ALGER FUNDS
Prospectus Date	rr_ProspectusDate	Mar. 01, 2019
Class P-2 Shares | Alger 25 Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>ALGER 25 FUND</b>
Objective [Heading]	rr_ObjectiveHeading	<b>Investment Objective</b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Alger 25 Fund seeks long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	<b>Fund Fees and Expenses</b>
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	<b>Shareholder Fees</b><br/>(fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	<b>Annual Fund Operating Expenses</b><br/>(expenses that you pay each year as a percentage of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	<b>Portfolio Turnover</b>
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 period since the Fund’s inception through October 31, 2018, the Fund’s portfolio turnover rate was 64.02% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	64.02%
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>Example</b>
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, that there is no performance adjustment to the management fee, and that the Fund’s operating expenses remain the same. The one-year example and the first two years 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. 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	<b>Principal Investment Strategy</b>
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, 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. Under normal circumstances, the Fund invests in a 25-stock portfolio of equity securities of companies of any market capitalization that Fred Alger Management, Inc. believes are undergoing Positive Dynamic Change. Equity securities include common or preferred stocks, or securities convertible into or exchangeable for equity securities, including warrants and rights. The Fund invests primarily in companies whose securities are traded on domestic exchanges. The Fund focuses its investments in companies fostering and benefiting from technological improvements, advancements or developments. In the opinion of Fred Alger Management, Inc., these companies use technology extensively to improve their business processes, applications and opportunities or seek to grow through technological developments and innovations. The Fund intends to invest a substantial portion of its assets in a smaller number of issuers. Generally the Fund will own approximately 25 holdings. As a result, the Fund is a non-diversified investment company, which means the performance results of any one position may have a greater impact on the Fund’s performance. The Fund’s portfolio manager 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. The Fund may invest a significant portion of its assets in securities of companies conducting business within a single sector, including the information technology, healthcare, consumer discretionary, and industrials sectors. The Fund can invest in foreign securities.
Risk [Heading]	rr_RiskHeading	<b>Principal Risks</b>
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. 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. Technology Companies Risk – The Fund may have a significant portion of its assets invested in securities of technology companies. Many technology companies have limited operating histories and prices of these 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. 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. Non-Diversification Risk – The Fund is a non-diversified investment company. Therefore, the Fund’s performance may be more vulnerable to changes in the market value of a single issuer and more susceptible to risks associated with a single economic, political, or regulatory occurrence than a fund that has a diversified portfolio. Small Number of Holdings Risk – Under normal circumstances, the Fund invests in a 25-stock portfolio. Therefore, the Fund’s performance may be more vulnerable to changes in the market value of a single issuer and more susceptible to risks associated with a single economic, political, or regulatory occurrence than a fund that has a higher number of holdings. 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 and Mid 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. 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. 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 may decrease more quickly than if the Fund had not borrowed. Fulcrum Fee Risk – With respect to Alger 25 Fund and Alger 35 Fund, the Manager is compensated, in part, for the Fund’s performance compared to the S&P 500 Index. As a result of this arrangement, the Manager’s compensation is maximized if the Fund significantly outperforms this benchmark, and the Manager receives the maximum performance adjustment to its management fee.
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.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	<b>Non-Diversification Risk</b> – The Fund is a non-diversified investment company. Therefore, the Fund’s performance may be more vulnerable to changes in the market value of a single issuer and more susceptible to risks associated with a single economic, political, or regulatory occurrence than a fund that has a diversified portfolio.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	<b>Performance</b>
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	No performance information will be presented until the Fund has been in operation for a full calendar year. Annual performance information gives some indication of the risks of an investment in the Fund by comparing the Fund’s performance with a broad measure of market performance. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. Updated performance will be available at www.alger.com within a reasonable amount of time after the Fund commences operations.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	No performance information will be presented until the Fund has been in operation for a full calendar year.
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 the Fund will perform in the future.
Class P-2 Shares | Alger 25 Fund | Class P-2
Risk/Return:	rr_RiskReturnAbstract
Shareholder Fees (fees paid directly from your investment)	rr_ShareholderFeeOther	none
Management Fees	rr_ManagementFeesOverAssets	0.55%	[1]
Shareholder Service Fees	rr_Component1OtherExpensesOverAssets	0.08%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.72%
Total Other Expenses	rr_OtherExpensesOverAssets	0.80%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.35%
Expense Reimbursement	rr_FeeWaiverOrReimbursementOverAssets	(0.54%)	[2]
Total Annual Fund Operating Expenses	rr_NetExpensesOverAssets	0.81%	[3]
1 Year	rr_ExpenseExampleYear01	$ 83
3 Years	rr_ExpenseExampleYear03	259
5 Years	rr_ExpenseExampleYear05	450
10 Years	rr_ExpenseExampleYear10	1,002
1 Year	rr_ExpenseExampleNoRedemptionYear01	83
3 Years	rr_ExpenseExampleNoRedemptionYear03	259
5 Years	rr_ExpenseExampleNoRedemptionYear05	450
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,002

[1] The management fee paid to Fred Alger Management, Inc. (the “Manager”) consists of a base fee at an annual rate of 0.55% of the Fund’s average daily net assets and a positive or negative performance adjustment of up to an annual rate of 0.25% based upon the Fund’s Class P-2 Shares performance relative to the S&P 500 Index, resulting in a minimum total fee of 0.30% and a maximum total fee of 0.80%.

[2] The Manager has contractually agreed to limit the Other Expenses of the Fund so that such expenses, excluding sub-transfer agent and shareholder servicing fees, never exceed 0.10% of average daily net assets. This commitment is currently expected to remain in place for the life of the Fund, can only be amended or terminated by agreement of the Fund’s Board of Trustees and the Manager, and will terminate automatically in the event of termination of the Investment Advisory Agreement between the Fund and the Manager. This expense reimbursement arrangement does not include dividend expense on short sales, borrowing costs, interest, taxes, brokerage and extraordinary expenses.

[3] After Expense Reimbursement