Label	Element	Value
Ariel Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Ariel Fund
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Ariel Fund’s fundamental objective is long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	FEES AND EXPENSES OF THE FUND
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes fees and expenses that you may pay if you buy and hold shares of Ariel Fund (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). Higher turnover rates may indicate higher transaction costs and may result in higher taxes when 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 23% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	23.00%
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	The example below illustrates the expenses you would pay on a $10,000 investment in the Fund. It assumes the Fund earned an annual return of 5% each year, the Fund’s operating expenses remain the same and that you redeem your shares at the end of each time period. The example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. Your actual expenses may be greater or less than the amounts shown.
Strategy [Heading]	rr_StrategyHeading	PRINCIPAL INVESTMENT STRATEGY
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund invests in small- and mid-capitalization (“small/mid cap”) undervalued companies that show strong potential for growth. The Fund invests primarily in equity securities of U.S. companies that have market capitalizations within the range of the companies in the Russell 2500TM Index, measured at the time of initial purchase. As of December 31, 2020, the market capitalizations of the companies in the Russell 2500 Index ranged from $43 million to $30.16 billion. The Fund may invest a portion of the portfolio outside (above or below) this market capitalization range. Also, the market capitalizations for the Fund’s portfolio companies may change over time, and the Fund is permitted to invest in (hold and purchase) a company even if its market capitalization moves outside the stated range. The essence of the Fund’s strategy is a combination of patience and stock selection. The Fund seeks to hold investments for a relatively long period of time—generally five years. However, the holding period may vary for any particular stock. The Fund seeks to invest in quality companies in industries in which Ariel Investments, LLC (“Ariel” or the “Adviser”) has expertise. This often results in the Fund investing a significant portion of its assets among fewer sectors than its benchmarks. The Fund buys a company when it is selling at a discount to Ariel’s estimate of its private market value. Quality companies typically share several attributes that Ariel believes will result in capital appreciation over time: high barriers to entry, sustainable competitive advantages, predictable fundamentals that allow for the potential for double-digit earnings growth (at time of initial purchase), skilled management teams, and solid financials. A high barrier to entry may exist where, for example, significant capital is required for new companies to enter a particular marketplace, thus giving companies already within the marketplace a perceived competitive advantage. Ariel’s strategy to focus on a limited number of companies and industries is designed to add value in areas in which we have expertise. We believe this approach creates a portfolio of well-researched stocks. As disciplined value investors, we make opportunistic purchases when great companies are temporarily out of favor—generally seeking to invest in companies that are trading at a low valuation relative to potential earnings and/or a low valuation relative to intrinsic worth. The primary reasons we will sell a stock are: (i) if its valuation reaches our determination of its private market value, (ii) if a better opportunity for investment presents itself, or (iii) if there are material adverse changes to a company’s fundamentals. In addition, the Adviser has adopted procedures to sell stocks that it views as substantially outside the strategy’s small/mid cap range. The Fund does not invest in companies whose primary source of revenue is derived from the production or sale of tobacco products or the manufacture of firearms. We believe these industries may be more likely to face shrinking growth prospects, litigation costs and legal liability that cannot be quantified. The Fund is a diversified fund that generally will hold between 25-45 securities in its portfolio.
Risk [Heading]	rr_RiskHeading	PRINCIPAL RISKS
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Although Ariel makes every effort to achieve the Fund’s objective of long-term capital appreciation, Ariel cannot guarantee it will attain that objective. You could lose money by investing in the Fund. The principal risks of investing in the Fund are:     •   Small/mid cap stocks held by the Fund could fall out of favor and returns would subsequently trail returns of the overall stock market. The performance of such stocks could also be more volatile. Small/mid cap stocks often have less predictable earnings, more limited product lines and markets, and more limited financial and management resources than large cap stocks.   •   The intrinsic value of the stocks in which the Fund invests may never be recognized by the broader market.   •   The Fund is often concentrated in fewer sectors than its benchmarks, and its performance may suffer if these sectors underperform the overall stock market.   •   Investing in equity securities is risky and subject to the volatility of the markets. Equity securities represent an ownership position in a company. The prices of equity securities fluctuate based on changes in the financial condition of their issuers and on market and economic conditions. Furthermore, when the stock market declines, most equity securities, even those issued by strong companies, often will decline in value. You should consider investing in the Fund if you are looking for long-term capital appreciation and are willing to accept the associated risks.
Risk Lose Money [Text]	rr_RiskLoseMoney	You could lose money by investing in the Fund.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	PERFORMANCE
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and the table below show two aspects of the Fund: variability and performance. The bar chart shows the variability of the Fund’s Investor Class annual total returns over time by showing changes in the Fund’s Investor Class performance from year to year. The table shows the Fund’s average annual total returns for certain time periods compared to the returns of the Russell 2500TM Value Index and the Russell 2500TM Index, which reflect the markets in which the Fund invests, and the S&P 500® Index, a broad measure of market performance. The bar chart and table provide some indication of the risks of investing in the Fund. To obtain updated performance information, visit the Fund’s website at arielinvestments.com or call 800.292.7435. The Fund’s past performance, before and after taxes, is not necessarily an indication of its future performance.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart shows the variability of the Fund’s Investor Class annual total returns over time by showing changes in the Fund’s Investor Class performance from year to year.
Performance Additional Market Index [Text]	rr_PerformanceAdditionalMarketIndex	The table shows the Fund’s average annual total returns for certain time periods compared to the returns of the Russell 2500TM Value Index and the Russell 2500TM Index, which reflect the markets in which the Fund invests, and the S&P 500® Index, a broad measure of market performance.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.292.7435
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	arielinvestments.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 its future performance.
Bar Chart [Heading]	rr_BarChartHeading	Total return for the year ended December 31
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter:   4Q ‘20     + 31.56%     Worst Quarter:   1Q ‘20     - 35.14%
Performance Table Heading	rr_PerformanceTableHeading	Average annual total returns as of December 31, 2020
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After tax returns are calculated using the highest historical 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	Actualafter-taxreturns depend on your tax situation and are not relevant if Fund shares are held intax-deferredarrangements, such as Individual Retirement Accounts.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-taxreturns are shown for the Investor Class only.After-taxreturns for the Institutional Class will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After tax returns are calculated using the highest historical individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-taxreturns depend on your tax situation and are not relevant if Fund shares are held intax-deferredarrangements, such as Individual Retirement Accounts.After-taxreturns are shown for the Investor Class only.After-taxreturns for the Institutional Class will vary.
Ariel Fund | Investor Class
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Maximum sales charge (load) imposed on reinvested dividends	rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther	none
Redemption fees	rr_RedemptionFeeOverRedemption	none
Management fees	rr_ManagementFeesOverAssets	0.59%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.20%
Total annual fund operating expenses	rr_ExpensesOverAssets	1.04%
1-Year	rr_ExpenseExampleYear01	$ 106
3-Year	rr_ExpenseExampleYear03	331
5-Year	rr_ExpenseExampleYear05	574
10-Year	rr_ExpenseExampleYear10	$ 1,271
2011	rr_AnnualReturn2011	(11.34%)
2012	rr_AnnualReturn2012	20.32%
2013	rr_AnnualReturn2013	44.68%
2014	rr_AnnualReturn2014	10.95%
2015	rr_AnnualReturn2015	(4.10%)
2016	rr_AnnualReturn2016	15.57%
2017	rr_AnnualReturn2017	15.88%
2018	rr_AnnualReturn2018	(13.67%)
2019	rr_AnnualReturn2019	24.67%
2020	rr_AnnualReturn2020	10.02%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2020
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	31.56%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2020
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(35.14%)
1-Year	rr_AverageAnnualReturnYear01	10.02%
5-Year	rr_AverageAnnualReturnYear05	9.66%
10-Year	rr_AverageAnnualReturnYear10	10.04%
Since Inception	rr_AverageAnnualReturnSinceInception	11.00%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 06, 1986
Ariel Fund | Institutional Class
Risk/Return:	rr_RiskReturnAbstract
Maximum sales charge (load) imposed on purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum deferred sales charge (load)	rr_MaximumDeferredSalesChargeOverOther	none
Maximum sales charge (load) imposed on reinvested dividends	rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther	none
Redemption fees	rr_RedemptionFeeOverRedemption	none
Management fees	rr_ManagementFeesOverAssets	0.59%
Distribution and service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.13%
Total annual fund operating expenses	rr_ExpensesOverAssets	0.72%
1-Year	rr_ExpenseExampleYear01	$ 74
3-Year	rr_ExpenseExampleYear03	230
5-Year	rr_ExpenseExampleYear05	401
10-Year	rr_ExpenseExampleYear10	$ 894
1-Year	rr_AverageAnnualReturnYear01	10.37%
5-Year	rr_AverageAnnualReturnYear05	9.99%
10-Year	rr_AverageAnnualReturnYear10	10.35%	[1]
Since Inception	rr_AverageAnnualReturnSinceInception	11.09%	[1]
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 06, 1986
Ariel Fund | return after taxes on distributions | Investor Class
Risk/Return:	rr_RiskReturnAbstract
1-Year	rr_AverageAnnualReturnYear01	8.57%
5-Year	rr_AverageAnnualReturnYear05	7.97%
10-Year	rr_AverageAnnualReturnYear10	8.48%
Since Inception	rr_AverageAnnualReturnSinceInception	9.43%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 06, 1986
Ariel Fund | return after taxes on distributions and sale of fund shares | Investor Class
Risk/Return:	rr_RiskReturnAbstract
1-Year	rr_AverageAnnualReturnYear01	6.80%
5-Year	rr_AverageAnnualReturnYear05	7.40%
10-Year	rr_AverageAnnualReturnYear10	7.96%
Since Inception	rr_AverageAnnualReturnSinceInception	9.13%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 06, 1986
Ariel Fund | Russell 2500™ Value Index (reflects no deductions for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1-Year	rr_AverageAnnualReturnYear01	4.88%
5-Year	rr_AverageAnnualReturnYear05	9.43%
10-Year	rr_AverageAnnualReturnYear10	9.33%
Since Inception	rr_AverageAnnualReturnSinceInception	10.76%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 06, 1986
Ariel Fund | Russell 2500™ Index (reflects no deductions for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1-Year	rr_AverageAnnualReturnYear01	19.99%
5-Year	rr_AverageAnnualReturnYear05	13.64%
10-Year	rr_AverageAnnualReturnYear10	11.97%
Since Inception	rr_AverageAnnualReturnSinceInception	10.98%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 06, 1986
Ariel Fund | S&P 500® Index (reflects no deductions for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1-Year	rr_AverageAnnualReturnYear01	18.40%
5-Year	rr_AverageAnnualReturnYear05	15.22%
10-Year	rr_AverageAnnualReturnYear10	13.88%
Since Inception	rr_AverageAnnualReturnSinceInception	10.78%
Inception Date	rr_AverageAnnualReturnInceptionDate	Nov. 06, 1986

[1] The inception date for the Institutional Class shares is December 30, 2011. Performance information for the Institutional Class prior to that date reflects the actual performance of the Fund’s Investor Class (and uses the actual expenses of the Investor Class, for such period of time), without any adjustments.