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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName WILLIAM BLAIR FUNDS
Prospectus Date rr_ProspectusDate Aug. 16, 2012
Institutional Class | William Blair International Leaders Fund
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading
SUMMARY
WILLIAM BLAIR INTERNATIONAL LEADERS FUND
Objective [Heading] rr_ObjectiveHeading INVESTMENT OBJECTIVE:
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The Fund seeks long-term capital appreciation.
Expense [Heading] rr_ExpenseHeading FEES AND EXPENSES:
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses that you may pay if you buy and hold Institutional Class 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 April 30, 2014
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). A higher portfolio turnover 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. Because the Fund is newly organized, portfolio turnover information is not available.
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates “Other Expenses” are estimated for the current fiscal year since the Fund did not commence operations until August 16, 2012.
Expense Example [Heading] rr_ExpenseExampleHeading Example:
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This example is intended to help you compare the cost of investing in Institutional Class shares of 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 and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year and the Fund’s operating expenses remain the same. The figures reflect the expense limitation for the first year. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading PRINCIPAL INVESTMENT STRATEGIES:
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock Under normal market conditions, the Fund invests primarily in a diversified portfolio of equity securities, including common stocks and other forms of equity investments (e.g., securities convertible into common stocks), issued by companies of all sizes domiciled outside the U.S, that the Advisor believes have above-average growth, profitability and quality characteristics. Under normal market conditions, the Fund typically holds a limited number of securities (i.e., 40-80 securities). The Advisor seeks investment opportunities in companies at different stages of development ranging from large, well-established companies to smaller companies at earlier stages of development, although the Fund’s assets primarily will be invested in securities of companies that have a float adjusted market capitalization greater than $3 billion. The Fund’s investments are normally allocated among at least six different countries and no more than 50% of the Fund’s equity holdings may be invested in securities of issuers in one country at any given time. Normally, the Fund’s investments will be divided among Continental Europe, the United Kingdom, Canada, Japan and the markets of the Pacific Basin. The Fund may invest the greater of 40% of its net assets or twice the emerging markets component of the MSCI All Country World Ex-U.S. Investable Market Index (IMI) (net) in emerging markets, which include every country in the world except the United States, Canada, Japan, Australia, New Zealand, Hong Kong, Singapore and most Western European countries.

In choosing investments, fundamental company analysis and stock selection are the Advisor’s primary investment criteria. The Advisor generally seeks equity securities, including common stocks, of companies that historically have had superior growth, profitability and quality relative to local markets and relative to companies within the same industry worldwide, and that are expected to continue such performance. Such companies generally will exhibit superior business fundamentals, including leadership in their field, quality products or services, distinctive marketing and distribution, pricing flexibility and revenue from products or services consumed on a steady, recurring basis. These business characteristics should be accompanied by management that is shareholder return-oriented and that uses conservative accounting policies. Companies with above-average returns on equity, strong balance sheets and consistent, above-average earnings growth will be the primary focus. Stock selection will take into account both local and global comparisons.

The Advisor will vary the Fund’s sector and geographic diversification based upon the Advisor’s ongoing evaluation of economic, market and political trends throughout the world. In making decisions regarding country allocation, the Advisor will consider such factors as the conditions and growth potential of various economies and securities markets, currency exchange rates, technological developments in the various countries and other pertinent financial, social, national and political factors.
Risk [Heading] rr_RiskHeading PRINCIPAL RISKS OF INVESTING:
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock Because the Fund invests most of its assets in equity securities of foreign companies, the primary risk is that the value of the equity securities it holds might decrease in response to the activities of those companies or market and economic conditions. In addition, there is the risk that individual securities may not perform as expected or a strategy used by the Advisor may fail to produce its intended result. Because the Fund may focus its investments in a limited number of securities, its performance may be more volatile than a fund that invests in a greater number of securities. Thus, the Fund’s returns will vary, and you could lose money by investing in the Fund. Foreign investments often involve additional risks, including political instability, differences in financial reporting standards and less stringent regulation of securities markets. Because the securities held by the Fund usually will be denominated in currencies other than the U.S. dollar, changes in foreign currency exchange rates may adversely affect the value of the Fund’s investments. The Fund is expected to incur operating expenses that are higher than those of mutual funds investing exclusively in U.S. equity securities due to the higher custodial fees associated with foreign securities investments. These foreign investment risks are magnified in less-established, emerging markets. In addition, the Fund may invest in the securities of smaller companies, which may be more volatile and less liquid than securities of large companies. In addition, smaller companies may be traded in low volumes. This can increase volatility and increase the risk that the Fund will not be able to sell the security on short notice at a reasonable price. To the extent the Fund invests a significant portion of its assets in one country, the Fund will be more vulnerable to the risks of adverse economic or political forces in that country.

The Fund involves a high level of risk and may not be appropriate for everyone.    You should only consider it for the aggressive portion of your portfolio. Separate accounts managed by the Advisor may invest in the Fund and, therefore, the Advisor at times may have discretionary authority over a significant portion of the assets invested in the Fund. In such instances, the Advisor’s decision to make changes to or rebalance its clients’ allocations in the separate accounts may substantially impact the Fund’s performance. The Fund is designed for long-term investors.
Risk Lose Money [Text] rr_RiskLoseMoney Thus, the Fund’s returns will vary, and you could lose money by investing in the Fund.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading FUND PERFORMANCE HISTORY:
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table showing the Fund’s annual returns and average annual total return are not included because the Fund does not have annual returns for a full calendar year.
Performance One Year or Less [Text] rr_PerformanceOneYearOrLess The bar chart and table showing the Fund’s annual returns and average annual total return are not included because the Fund does not have annual returns for a full calendar year.
Institutional Class | William Blair International Leaders Fund | Institutional Class
  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Redemption Fee (as a percentage of amount redeemed, for shares held 60 days or less) rr_RedemptionFeeOverRedemption none
Management Fee rr_ManagementFeesOverAssets 0.95%
Distribution (Rule 12b-1) Fee rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.29% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.24% [2]
Fee Waiver and/or Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.19%
Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement rr_NetExpensesOverAssets 1.05%
1 Year rr_ExpenseExampleYear01 107
3 Years rr_ExpenseExampleYear03 375
[1] "Other Expenses" are estimated for the current fiscal year since the Fund did not commence operations until August 16, 2012.
[2] The Advisor has entered into a contractual agreement with the Fund to waive fees and/or reimburse expenses in order to limit the Fund's Institutional Class operating expenses (excluding interest, taxes, brokerage commissions, acquired fund fees and expenses, other investment-related costs and extraordinary expenses, such as litigation and other expenses not incurred in the ordinary course of the Fund's business) to 1.05% of average daily net assets until April 30, 2014. The Advisor may not terminate this arrangement prior to April 30, 2014 unless the investment advisory agreement is terminated. The Advisor is entitled to reimbursement for a period of three years subsequent to the Fund's Commencement of Operations on August 16, 2012 for previously waived fees and reimbursed expenses to the extent that the Fund's expense ratio is below the expense limitation.