The Fund incurs transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate 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. During the most recent fiscal year, the Fund's portfolio turnover rate was 148% of the average value of its portfolio.

Portfolio Turnover, Rate

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148.00%

Expenses Restated to Reflect Current [Text]

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The expense information in the fee table has been restated to reflect that effective September 1, 2011, the Fund entered into a new transfer agency agreement

Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]

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Total Annual Fund Operating Expenses shown will not correlate to the Fund's ratios of expenses to average net assets appearing in the Financial Highlights tables, which do not include Acquired Fund Fees and Expenses.

Expense Example [Heading]

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Example

Expense Example Narrative [Text Block]

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This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other 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 that the Fund's operating expenses are as shown in the table and remain the same. The costs in the one-year example and for the first year of the three-, five-, and ten-year examples reflect the Adviser's agreement to waive fees and reimburse expenses through July 6, 2013. Although your actual costs and returns may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

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Principal Investment Strategies

Strategy Narrative [Text Block]

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Under normal circumstances, the Fund invests at least 80% of its assets in municipal securities, the income from which is exempt from federal income tax (including the federal alternative minimum tax (AMT)). The Fund normally maintains an average dollar-weighted effective maturity of one year or less. Effective maturity takes into account the possibility that a bond may have prepayments or may be called by the issuer before its stated maturity date.

The Fund invests primarily in municipal securities within the investment grade category (i.e., rated BBB or Baa, or higher, or unrated and considered by the Adviser to be comparable in quality) at the time of purchase. The Fund also may invest up to 20% of its assets in municipal securities that are below investment grade (also known as high yield securities or "junk bonds") rated BB or Ba or lower or unrated and considered by the Adviser to be comparable in quality at the time of purchase. Municipal securities include fixed and floating rate debt obligations of states, territories and possessions of the U.S. and political subdivisions and financing authorities of these entities that provide income exempt from federal income tax (including federal AMT). Fund investments are selected after assessing factors such as the cyclical trend in interest rates, the shape of the municipal yield curve, tax rates, sector valuation and municipal bond supply factors.

Risk [Heading]

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Principal Risks

Risk Narrative [Text Block]

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The Fund cannot assure that it will achieve its investment objective. An investment in the Fund is not a deposit of BMO Harris Bank N.A. or any of its affiliates and is not insured or guaranteed by the FDIC or any other government agency. The net asset value (NAV) of the Fund will vary and you could lose money by investing in the Fund. In addition, the Fund is subject to the following risks.

Interest Rate Risk. Prices of fixed income securities rise and fall in response to changes in the interest rate paid by similar securities. Generally, when interest rates rise, prices of fixed income securities fall. Interest rate changes have a greater effect on the price of fixed income securities with longer maturities.

Credit Risk. Credit risk is the possibility that an issuer will default on a security by failing to pay interest or principal when due. If an issuer defaults, the Fund may lose money. Lower credit ratings correspond to higher credit risk. Bonds rated lower than BBB or Baa have speculative characteristics.

Call Risk. If the securities in which the Fund invests are redeemed by the issuer before maturity (or "called"), the Fund may have to reinvest the proceeds in securities that pay a lower interest rate, which may decrease the Fund's yield. This will most likely happen when interest rates are declining.

Liquidity Risk. Liquidity risk refers to the possibility that the Fund may not be able to sell or buy a security or close out an investment contract at a favorable price or time. Consequently, the Fund may have to accept a lower price to sell a security, sell other securities to raise cash or give up an investment opportunity, any of which could have a negative effect on the Fund's performance. Infrequent trading of securities also may lead to an increase in their price volatility.

Municipal Securities Risk. Municipal bonds are subject to risks based on many factors, including economic and regulatory developments, changes or proposed changes in the federal and state tax structure, deregulation, court rulings and other factors. The value of municipal securities may be affected more by supply and demand factors or the creditworthiness of the issuer than by market interest rates. Repayment of municipal securities depends on the ability of the issuer or project backing such securities to generate taxes or revenues. There is a risk that the interest on an otherwise tax-exempt municipal security may be subject to federal income tax.

High Yield Securities Risks. High yield securities, also referred to as "junk bonds" or non-investment grade securities, are debt securities rated lower than BBB by Standard & Poor's or Baa by Moody's Investors Service. These securities tend to be more sensitive to economic conditions than are higher-rated securities, generally involve more credit risk than securities in the higher-rated categories and are predominantly considered to be speculative. The issuers of high yield securities are typically more leveraged, and the risk of loss due to default by an issuer of high yield securities is significantly greater than issuers of higher-rated securities because such securities are generally unsecured and are often subordinated to other creditors. The Fund may have difficulty disposing of certain high yield securities because there may be a thin trading market for such securities.

Management Risk. The Adviser's judgments about the attractiveness, value and potential appreciation of the Fund's investments may prove to be incorrect. Accordingly, there is no guarantee that the investment techniques used by the Fund's managers will produce the desired results.

Sector Risks. The Fund may invest its assets in municipal securities that finance similar projects, such as those relating to education, health care, transportation and utilities. To the extent the Fund invests its assets in a particular sector, the Fund's performance may be more susceptible to any economic, business or other developments that generally affect that sector.

Investments in Other Investment Companies Risks. The Fund may invest in securities issued by other investment companies, including exchange-traded funds. By investing in another investment company, there is a risk that the value of the underlying securities of the investment company may decrease. The Fund will also bear its proportionate share of the other investment company's fees and expenses.

Portfolio Turnover Risks. A high portfolio rate (100% or more) may result in the realization and distribution to shareholders of a greater amount of capital gains than if the Fund had a low portfolio turnover rate. Therefore, you may have higher tax liability. High portfolio turnover may also result in higher transaction costs, which may negatively affect Fund performance.

Risk Lose Money [Text]

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The net asset value (NAV) of the Fund will vary and you could lose money by investing in the Fund.

Risk Not Insured Depository Institution [Text]

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An investment in the Fund is not a deposit of BMO Harris Bank N.A. or any of its affiliates and is not insured or guaranteed by the FDIC or any other government agency.

Bar Chart and Performance Table [Heading]

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Fund Performance

Performance Narrative [Text Block]

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The bar chart and table show the historical performance of the Fund's shares and provide some indication of the risks of investing in the Fund. The bar chart shows how the Fund's total returns before taxes have varied from year to year, while the table compares the Fund's average annual total returns to the returns of a broad measure of market performance and an index of funds with similar investment objectives. Please keep in mind that past performance, before and after taxes, does not represent how the Fund will perform in the future. Investors may obtain updated performance information for the Fund at www.bmofundsus.com.

Performance Information Illustrates Variability of Returns [Text]

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The bar chart shows how the Fund's total returns before taxes have varied from year to year, while the table compares the Fund's average annual total returns to the returns of a broad measure of market performance and an index of funds with similar investment objectives.

Performance Availability Website Address [Text]

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www.bmofundsus.com

Performance Past Does Not Indicate Future [Text]

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Please keep in mind that past performance, before and after taxes, does not represent how the Fund will perform in the future.

Bar Chart [Heading]

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Class Y—Annual Total Returns (calendar year 2010)

Bar Chart Closing [Text Block]

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The return for the Class Y shares of the Fund from January 1, 2011 through September 30, 2011 was 1.29%.

During the periods shown in the bar chart for the Fund:


	Quarter Ended		Returns
Best quarter		9/30/2010		0.54	%
Worst quarter		12/31/2010		0.05	%

Performance Table Heading

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Average Annual Total Returns through 12/31/10

Performance Table Uses Highest Federal Rate

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After-tax returns are calculated using the highest historical individual marginal federal income tax rates and do not reflect the effect of any applicable state and local taxes.

Performance Table Not Relevant to Tax Deferred

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After-tax returns shown are not relevant to investors holding shares through tax-deferred programs, such as IRAs or 401(k) plans.

Performance Table One Class of after Tax Shown [Text]

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After-tax returns are shown only for Class Y, and after-tax returns for Class I will vary.

Performance Table Narrative

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After-tax returns are calculated using the highest historical individual marginal federal income tax rates and do not reflect the effect of any applicable state and local taxes. Actual after-tax returns depend on an investor's tax situation and may differ from those shown. After-tax returns shown are not relevant to investors holding shares through tax-deferred programs, such as IRAs or 401(k) plans. After-tax returns are shown only for Class Y, and after-tax returns for Class I will vary.

The Barclays Capital 1-Year Municipal Bond Index (BMB 1) is the 1-year component of the Barclays Capital Municipal Bond Index, which is an unmanaged index composed of long-term tax-exempt bonds with a minimum credit rating of Baa.

The iMoneyNet Money Fund Report Tax-Free National Retail Index (IMNTFNR) is an average of money funds with investment objectives similar to that of the Fund.

The Lipper Short Municipal Debt Fund Index (LSMDI) is an average of the 30 largest mutual funds in this Lipper category.

BMO Ultra Short Tax-Free Fund | Blended Benchmark (reflects no deduction for fees, expenses or taxes)

Risk/Return:

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1 Year

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0.60%

^[4]

Since Commencement of Operations

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0.90%