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Ariel Focus Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Ariel Focus Fund
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
Ariel Focus 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 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 Oct. 01, 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).  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 16% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 16.00%
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
The example below illustrates the expenses you would pay on a $10,000 investment in Ariel Focus 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 reflects contractual fee waivers and expense reimbursements effective through September 30, 2017.  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
Ariel Focus Fund invests primarily in equity securities of companies of any size in order to provide investors access to superior opportunities in companies of all market capitalizations.  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 three to five years or more.  However, the holding period may vary for any particular stock.  The Fund seeks to invest primarily in high quality companies that are trading at low multiples of their actual or projected earnings.  The Fund only buys when Ariel believes that these companies are selling at excellent values.

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 double digit earnings growth, 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 it has 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 is: (i) if its valuation reaches our private market value, as determined by Ariel, (ii) if a better opportunity for investment presents itself, or (iii) if there are material changes to a company’s fundamentals.

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 handguns.  We believe these industries are more likely to face shrinking growth prospects, draining litigation costs and legal liability that cannot be quantified.

Ariel Focus Fund is a non-diversified fund, which means that the Fund could own as few as 12 securities in its portfolio, but will generally own 20–30 securities.
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 this Fund.  The principal risks of investing in the Fund are:

●
As the Fund holds relatively few stocks, a fluctuation in one stock could significantly affect overall performance.

●
Small and medium capitalization stocks held by the Fund could fall out of favor and returns would subsequently trail returns from the overall stock market.  The performance of such stocks could also be more volatile.  Small and medium capitalization stocks often have less predictable earnings, more limited product lines and markets, and more limited financial and management resources than large capitalization stocks.

●
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.

●
The intrinsic value of the stocks in which the Fund invests may never be recognized by the broader market.

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 this Fund.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading PERFORMANCE
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The bar chart and the table on the next page 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 S&P 500® Index, a broad measure of market performance, and an index that reflects the market sectors in which the Fund invests.  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 and table provide some indication of the risks of investing in the Fund.
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:
2Q ‘09
+ 22.87%
 
Worst Quarter:
4Q ‘08
- 26.82%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 22.87%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (26.82%)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deductions for fees, expenses or taxes)
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 Actual after-tax returns depend on your tax situation and are not relevant if Fund shares are held in tax-deferred arrangements, such as IRAs.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns are shown for the Investor Class only. After-tax returns for the Institutional Class will vary.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock
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-tax returns depend on your tax situation and are not relevant if Fund shares are held in tax-deferred arrangements, such as IRAs.  After-tax returns are shown for the Investor Class only.  After-tax returns for the Institutional Class will vary.
Caption rr_AverageAnnualReturnCaption Average annual total returns as of December 31, 2015
Ariel Focus Fund | S&P 500® Index (reflects no deductions for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel S&P 500® Index (reflects no deductions for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 1.38%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 12.57%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.31%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.52%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jun. 30, 2005
Ariel Focus Fund | Russell 1000® Value Index (reflects no deductions for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Russell 1000® Value Index (reflects no deductions for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (3.83%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 11.27%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.16%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 6.36%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jun. 30, 2005
Ariel Focus Fund | Investor Class  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Maximum sales charge (load) imposed on reinvested dividends rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther none
Redemption fees rr_RedemptionFeeOverRedemption none
Management fees rr_ManagementFeesOverAssets 0.65%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.47%
Total annual fund operating expenses rr_ExpensesOverAssets 1.37% [1]
Less fee waiver or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.37%)
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 1.00%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 102
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 359
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 677
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,580
Annual Return 2006 rr_AnnualReturn2006 11.58%
Annual Return 2007 rr_AnnualReturn2007 (2.54%)
Annual Return 2008 rr_AnnualReturn2008 (35.09%)
Annual Return 2009 rr_AnnualReturn2009 33.46%
Annual Return 2010 rr_AnnualReturn2010 11.13%
Annual Return 2011 rr_AnnualReturn2011 (5.21%)
Annual Return 2012 rr_AnnualReturn2012 11.42%
Annual Return 2013 rr_AnnualReturn2013 39.37%
Annual Return 2014 rr_AnnualReturn2014 9.92%
Annual Return 2015 rr_AnnualReturn2015 (15.39%)
Label rr_AverageAnnualReturnLabel Investor Class return before taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (15.39%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 6.48%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 3.66%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 4.03%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jun. 30, 2005
Ariel Focus Fund | Investor Class | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Investor Class return after taxes on distributions
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (17.77%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 4.99%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 2.85%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 3.22%
Ariel Focus Fund | Investor Class | After Taxes on Distributions and Sale of Fund Shares  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Investor Class return after taxes on distributions and sale of fund shares
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (6.60%)
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 5.17%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 2.94%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 3.23%
Ariel Focus Fund | Institutional Class  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Maximum sales charge (load) imposed on reinvested dividends rr_MaximumSalesChargeOnReinvestedDividendsAndDistributionsOverOther none
Redemption fees rr_RedemptionFeeOverRedemption none
Management fees rr_ManagementFeesOverAssets 0.65%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.42%
Total annual fund operating expenses rr_ExpensesOverAssets 1.07% [1]
Less fee waiver or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.32%)
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.75%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 77
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 275
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 526
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,246
Label rr_AverageAnnualReturnLabel Institutional Class return before taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 (15.19%) [2]
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 6.69% [2]
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 3.77% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 4.13% [2]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2011 [2]
[1] The Adviser is contractually obligated to waive fees or reimburse expenses (excluding acquired fund fees and expenses, brokerage, interest, taxes, distribution plan expenses, and extraordinary items) in order to limit Ariel Focus Fund's total annual fund operating expenses to 1.00% of net assets for the Investor Class and 0.75% of net assets for the Institutional Class through the end of the fiscal year ending September 30, 2017. No termination of this agreement by either the Fund's Board of Trustees or the Adviser may be effective until, at the earliest, October 1, 2017.
[2] 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.