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Risk/Return: rr_RiskReturnAbstract  
ProspectusDate rr_ProspectusDate Dec. 30, 2011
Ariel Fund (Prospectus Summary) | Ariel Fund
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return, Heading rr_RiskReturnHeading Ariel Fund
Investment Objective, Heading rr_ObjectiveHeading Investment objective
investment Objective, Primary rr_ObjectivePrimaryTextBlock 
Ariel Fund pursues long-term capital appreciation by investing in undervalued
companies that show strong potential for growth.
Expense, Heading rr_ExpenseHeading Fees and expenses of the Fund
Expense, Narrative rr_ExpenseNarrativeTextBlock 
The table below describes fees and expenses that you may pay if you buy and hold
shares of the Fund.  You do not pay a sales charge or load when you buy or sell
shares.
Operating Expenses, Caption 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 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 29% of the
average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 29.00%
Other Expenses, New Fund, Based on Estimates rr_OtherExpensesNewFundBasedOnEstimates These expenses are based on estimated amounts for the current fiscal year.
Expense Example, Narrative rr_ExpenseExampleNarrativeTextBlock 
The example below illustrates the expenses you would pay on a $10,000 investment
in Ariel 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.
Investment Strategy, Heading rr_StrategyHeading Principal investment strategy
Investment Strategy, Narrative rr_StrategyNarrativeTextBlock 
Ariel Fund invests primarily in common stocks of U.S. companies and the Fund
generally will have a weighted average market capitalization between $1 billion
and $7.5 billion.  The Fund will not hold stocks that fall within the top two
quintiles of the Russell U.S. equity indexes (a comprehensive representation of
market-cap weighted security indexes of the investable U.S. equity market) and
if such stock falls outside the parameters, it will be sold by the end of the
following quarter.

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.

The Fund seeks to invest in quality companies in industries in which Ariel
Investments, LLC ("Ariel" or the "Adviser") has expertise including the
financial services and consumer  discretionary sectors.  The Fund only buys when
Ariel believes that these businesses 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.  Ariel's
strategy to focus on a limited number of names 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.  We will sell a stock if its valuation reaches our private
market value, as determined by the Adviser, or if there are material changes to
a company's fundamentals.

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 Fund is a diversified fund that will generally hold between 30-50
securities in its portfolio.
Risk, Heading rr_RiskHeading Principal risks
Risk, Narrative 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:

o 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.

o The general level of stock prices could decline.

o The Fund often invests a significant portion of its assets in companies within
  the financial services and consumer discretionary sectors and its performance
  may suffer if these sectors underperform the overall stock 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 rr_RiskLoseMoney You could lose money by investing in this Fund.
Bar Chart and Performance Table, Heading rr_BarChartAndPerformanceTableHeading Performance
Performance, Narrative rr_PerformanceNarrativeTextBlock 
The bar chart below and the table on the following 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
Investor Class 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 rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart below and the table on the following page show two aspects of the Fund: variability and performance.
Performance, Availability Phone Number rr_PerformanceAvailabilityPhone 800.292.7435
Performance, Availability Website Address rr_PerformanceAvailabilityWebSiteAddress arielinvestments.com
Performance, Past Does Not Indicate Future 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 rr_BarChartClosingTextBlock 
Best Quarter:  2Q'09 +34.75%

Worst Quarter: 4Q'08 -34.28%
Performance Table, Market Index Changed rr_PerformanceTableMarketIndexChanged One of the Fund's benchmarks has been changed from the Russell 2500 Index to the Russell 2000 Value Index as the Adviser believes the new index is more indicative of the market capitalization and style profile of the Fund.
Index No Deduction for Fees, Expenses, Taxes 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 Individual Retirement Accounts (IRAs).
Performance Table, Closing 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
Individual Retirement Accounts (IRAs).  One of the Fund's benchmarks has been
changed from the Russell 2500 Index to the Russell 2000 Value Index as the
Adviser believes the new index is more indicative of the market capitalization
and style profile of the Fund.  The inception date for the Fund's Class I shares
is December 30, 2011.
Average Annual Returns, Caption rr_AverageAnnualReturnCaption Average Annual Total Returns As of December 31, 2010
Ariel Fund (Prospectus Summary) | Ariel Fund | Investor Class
  
Risk/Return: rr_RiskReturnAbstract  
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 34.75%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2008
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (34.28%)
Ariel Fund | S&P 500 Index
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel S&P 500 Index (reflects no deductions for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 15.06%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 2.29%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 1.41%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 9.51%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 06, 1986
Ariel Fund | Russell 2000 Value Index
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Russell 2000 Value Index (reflects no deductions for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 24.50%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 3.52%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 8.42%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 10.73%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 06, 1986
Ariel Fund | Russell 2500 Value Index
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Russell 2500 Value Index (reflects no deductions for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 24.82%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 3.85%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 8.53%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 11.34%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 06, 1986
Ariel Fund | Russell 2500 Index
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Russell 2500 Index (reflects no deductions for fees, expenses or taxes)
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 26.71%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 4.86%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.98%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 10.57%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 06, 1986
Ariel Fund | Investor Class
  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.59%
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.20%
Total annual operating expenses rr_ExpensesOverAssets 1.04%
Expense Example, With Redemption, 1 Year rr_ExpenseExampleYear01 106
Expense Example, With Redemption, 3 Years rr_ExpenseExampleYear03 331
Expense Example, With Redemption, 5 Years rr_ExpenseExampleYear05 574
Expense Example, With Redemption, 10 Years rr_ExpenseExampleYear10 1,271
Annual Return 2001 rr_AnnualReturn2001 14.21%
Annual Return 2002 rr_AnnualReturn2002 (5.18%)
Annual Return 2003 rr_AnnualReturn2003 28.04%
Annual Return 2004 rr_AnnualReturn2004 21.97%
Annual Return 2005 rr_AnnualReturn2005 0.92%
Annual Return 2006 rr_AnnualReturn2006 10.35%
Annual Return 2007 rr_AnnualReturn2007 (1.70%)
Annual Return 2008 rr_AnnualReturn2008 (48.25%)
Annual Return 2009 rr_AnnualReturn2009 63.42%
Annual Return 2010 rr_AnnualReturn2010 25.97%
Average Annual Returns, Label rr_AverageAnnualReturnLabel Investor Class Return Before Taxes
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 25.97%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 2.93%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 7.03%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 11.39%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 06, 1986
Ariel Fund | Investor Class | After Taxes on Distributions
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Investor Class Return After Taxes on Distributions
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 25.97%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 2.41%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.49%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 9.82%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 06, 1986
Ariel Fund | Investor Class | After Taxes on Distributions and Sales
  
Risk/Return: rr_RiskReturnAbstract  
Average Annual Returns, Label rr_AverageAnnualReturnLabel Investor Class Return After Taxes on Distributions and Sale of Fund Shares
Average Annual Returns, 1 Year rr_AverageAnnualReturnYear01 16.88%
Average Annual Returns, 5 Years rr_AverageAnnualReturnYear05 2.47%
Average Annual Returns, 10 Years rr_AverageAnnualReturnYear10 6.10%
Average Annual Returns, Since Inception rr_AverageAnnualReturnSinceInception 9.55%
Average Annual Returns, Inception Date rr_AverageAnnualReturnInceptionDate Nov. 06, 1986
Ariel Fund | Institutional Class (Class I)
  
Risk/Return: rr_RiskReturnAbstract  
Management fees rr_ManagementFeesOverAssets 0.59% [1]
Distribution and service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.20% [1]
Total annual operating expenses rr_ExpensesOverAssets 0.79% [1]
Expense Example, With Redemption, 1 Year rr_ExpenseExampleYear01 81
Expense Example, With Redemption, 3 Years rr_ExpenseExampleYear03 252
Expense Example, With Redemption, 5 Years rr_ExpenseExampleYear05 439
Expense Example, With Redemption, 10 Years rr_ExpenseExampleYear10 978
[1] The inception date for the Class I shares is December 30, 2011. These expenses are based on estimated amounts for the current fiscal year.