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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Ariel Discovery Fund
Objective [Heading] rr_ObjectiveHeading Investment objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
Ariel Discovery Fund pursues 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 2014-09-30
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 33% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 33.00%
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
The example below illustrates the expenses you would pay on a $10,000 investment in Ariel Discovery 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 through September 30, 2014.  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 Discovery Fund invests primarily in common stocks of companies with market capitalizations under $2 billion, measured at the time of initial purchase.  Over time, the market capitalizations for the Fund’s portfolio companies may change.

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.  As long as a portfolio company otherwise meets the Fund’s investment criteria and style, increased capitalization does not prevent the Fund from holding or buying more shares.

The Fund seeks to invest in companies which trade at low price-to-book ratios and have strong balance sheets.  The Fund only buys when Ariel believes these businesses are selling at deep discounts to their intrinsic value.

Individual company valuations will be based on assets and/or earnings power.  Flexibility in determining the proper valuation metric for a given company will be a key component of the Fund’s strategy.  We believe this approach creates a portfolio of well-researched stocks.  The Fund is managed on a dynamic basis as companies which approach fair value are likely to be sold and replaced by those with deeper discounts to intrinsic value.  The Fund may hold cash when values are difficult to identify.

The Fund does not invest in corporations 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 Discovery Fund is a diversified fund that generally will not hold more than 50 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 this Fund.  The principal risks are:

v
Small 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 capitalization companies often have less predictable earnings, more limited product lines and markets and more limited financial and management resources.

v
The general level of stock prices could decline.

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 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 S&P 500 Index, a broad measure of market performance, and indices that reflect 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 the table below show two aspects of the Fund: variability and 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: 1Q '12 +11.32%
Worst Quarter: 4Q '12 -1.98%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2012
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 11.32%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2012
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (1.98%)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes (reflects no deductions for fees, expenses or taxes)
Caption rr_AverageAnnualReturnCaption Average Annual Total Returns As of December 31, 2012
S&P 500® Index (reflects no deductions for fees, expenses or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1-Year rr_AverageAnnualReturnYear01 16.00%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 7.92%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jan. 31, 2011
Russell 2000® Value Index (reflects no deductions for fees, expenses or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1-Year rr_AverageAnnualReturnYear01 18.05%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 5.85%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jan. 31, 2011
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 1.00%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 3.93%
Total annual operating expenses rr_ExpensesOverAssets 5.18% [1]
Less fee waiver or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (3.68%)
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 1.50%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 153
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 471
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,577
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 4,324
Annual Return 2012 rr_AnnualReturn2012 15.65%
Average Annual Returns, 1-Year rr_AverageAnnualReturnYear01 15.65%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 2.17%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Jan. 31, 2011
Investor Class | After Taxes on Distributions
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1-Year rr_AverageAnnualReturnYear01 15.65%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 2.17%
Investor Class | After Taxes on Distributions and Sale of Fund Shares
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, 1-Year rr_AverageAnnualReturnYear01 10.17%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 1.85%
Institutional Class (Class I)
  
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 1.00%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 3.78%
Total annual operating expenses rr_ExpensesOverAssets 4.78% [1]
Less fee waiver or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets (3.53%)
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 1.25%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 127
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 394
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 1,428
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 4,032
Average Annual Returns, 1-Year rr_AverageAnnualReturnYear01 16.09% [2]
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 2.38% [2]
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Dec. 30, 2011 [2]
[1] The Adviser is contractually obligated to waive fees or reimburse expenses in order to limit Ariel Discovery Fund's total annual operating expenses to 1.50% of net assets for the Investor Class and 1.25% of net assets for Class I through the end of the fiscal year ending September 30, 2014. No termination of this agreement by either the Board of Trustees or the Adviser may be effective until, at the earliest, October 1, 2014.
[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.