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FlexShares Real Assets Allocation Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading FlexShares® Real Assets Allocation Index Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
The Fund seeks investment results that correspond generally to the price and yield performance, before fees and expenses, of the Northern Trust Real Assets Allocation IndexSM (the “Underlying Index”).
Expense [Heading] rr_ExpenseHeading Fees And Expenses Of The Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock
This table describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund. Under the Fund’s Investment Advisory Agreement, the Fund is responsible for the following other expenses: interest expenses, brokerage commissions and other trading expenses, fees and expenses of the independent trustees and their independent legal counsel, taxes and other extraordinary costs such as litigation and other expenses not incurred in the ordinary course of business. You will also incur usual and customary brokerage commissions and fees to financial intermediaries when buying or selling shares of the Fund in the secondary market, which are not reflected in the example that follows:
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses that you pay eachyear as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination March 1, 2026
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 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. Portfolio turnover may vary from year to year, as well as within a year. During the most recent fiscal year, the Fund’s portfolio turnover rate was 28% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 28.00%
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text] rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees The “Total Annual Fund Operating Expenses” may not correlate to the ratio of expenses to average net assets as reported in the "Financial Highlights" section of the Prospectus, which reflects the operating expenses of the Fund and does not include “Acquired Fund Fees and Expenses.”
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
The following 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 sell 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 remain the same (taking into account the expense reimbursement arrangement for one 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
The Fund is a fund of funds and seeks its investment objective by investing primarily in the shares of other FlexShares® ETFs (each an “Underlying Fund” and together, the “Underlying Funds”) that are eligible for inclusion in the Underlying Index and listed below, rather than in securities of individual companies. The Underlying Funds themselves seek investment results corresponding to their own respective underlying indexes. The Underlying Funds invest primarily in separate sets of securities representing or providing exposures to global “real assets.” Real assets are defined by NTI, acting in its capacity as index provider (the “Index Provider”), as physical or tangible assets. Examples of real assets include but are not limited to
commodities, precious metals, oil, and real estate. Each Underlying Fund has its own risk profile and will contribute differently to the overall risk profile of the Fund. Each of the Underlying Funds invests in equity securities that are traded in or are issued by companies domiciled in global developed or emerging markets (including the U.S.). Certain of the Underlying Funds may invest in publicly-traded units of master limited partnerships (“MLPs”) and real estate investment trusts (“REITs”).
The Underlying Index is designed to reflect the performance of an optimized allocation to the Underlying Funds that is intended to provide exposures to certain real assets and minimize overall volatility of investment in the Underlying Funds.
As of December 31, 2024, the Underlying Index was comprised of the following Underlying Funds, each of which invests primarily in equity securities: a 50.05% weight to FlexShares® STOXX® Global Broad Infrastructure Index Fund, a 9.76% weight to FlexShares® Morningstar Global Upstream Natural Resources Index Fund and a 40.19% weight to FlexShares® Global Quality Real Estate Index Fund. The Underlying Index was constituted and is rebalanced at least annually using a rules-based methodology. The Underlying Index may be rebalanced semi-annually if the volatility of investment in the Underlying Funds surpasses a minimum threshold. The weights of each Underlying Fund, and the degree to which each Underlying Fund represents exposure to certain real assets, may change over time as the Underlying Index and the underlying indexes of the Underlying Funds are reconstituted and/or rebalanced (which may be quarterly) and their component securities fluctuate in value. The Fund generally reconstitutes and/or rebalances its portfolio in accordance with the Underlying Index.
NTI uses a “passive” or indexing approach to try to achieve the Fund’s investment objective. Unlike many investment companies, the Fund does not try to “beat” the index it tracks and does not seek temporary defensive positions when markets decline or appear overvalued.
NTI generally intends to replicate the constituent securities of the Fund’s Underlying Index. “Replication” is an indexing strategy in which a fund invests in substantially all of the securities in its underlying index in approximately the same proportions as in the underlying index. NTI may use a “representative sampling” strategy in certain circumstances, such as when it may not be possible or practicable to fully implement a replication strategy. Representative sampling is investing in a representative sample of securities that collectively has an investment profile similar to the Underlying Index.
Under normal circumstances, the Fund will invest at least 80% of its total assets (exclusive of collateral held from securities lending) in the securities of the Underlying Index. The Fund may also invest up to 20% of its assets in cash and cash equivalents, including shares of money market funds advised by NTI or its affiliates, futures contracts and options on futures contracts, as well as securities not included in the Underlying Index, but which NTI believes will help the Fund track its Underlying Index.
The Underlying Index is created and sponsored by NTI. NTI also serves as the investment adviser to the Fund and the Underlying Funds. NTI, as the Index Provider, determines the composition and relative weightings of the securities in the Underlying Index and publishes information regarding the market value of the Underlying Index.
The Fund may lend securities representing up to one-third of the value of the Fund’s total assets (including the value of the collateral received).
From time to time the Fund may focus its investments (i.e., invest more than 15% of its total assets) in one or more particular countries or geographic regions.
Industry Concentration Policy. The Fund will concentrate its investments (i.e., hold 25% or more of its total assets) in a particular industry or group of industries to approximately the same extent that the Underlying Index is concentrated. As of December 31, 2024, the Underlying Index was concentrated in real assets-related companies in the real estate, natural resources and infrastructure sectors. The components of the Underlying Index, and the degree to which these components represent certain industries or sectors, may change over time.
Strategy Portfolio Concentration [Text] rr_StrategyPortfolioConcentration Industry Concentration Policy. The Fund will concentrate its investments (i.e., hold 25% or more of its total assets) in a particular industry or group of industries to approximately the same extent that the Underlying Index is concentrated. As of December 31, 2024, the Underlying Index was concentrated in real assets-related companies in the real estate, natural resources and infrastructure sectors. The components of the Underlying Index, and the degree to which these components represent certain industries or sectors, may change over time.
Risk [Heading] rr_RiskHeading Principal Risks
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Fund Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The bar chart and table that follow show how the Fund has performed on a calendar year basis and provide an indication of the risks of investing in the Fund by showing (A) changes in the Fund’s performance from year to year, (B) how the Fund’s average annual returns compare with those of a broad measure of market performance, and (C) an additional index with characteristics relevant to the Fund. Past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future. Updated information on the Fund’s performance results can be obtained by visiting flexshares.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table that follow show how the Fund has performed on a calendar year basis and provide an indication of the risks of investing in the Fund by showing (A) changes in the Fund’s performance from year to year, (B) how the Fund’s average annual returns compare with those of a broad measure of market performance, and (C) an additional index with characteristics relevant to the Fund.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress flexshares.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture Past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Calendar Year Total Returns
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
For the periods in the bar chart above:
Best Quarter (3/31/2019): 14.27%
Worst Quarter (3/31/2020): -23.50%
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for the periods ended December 31, 2024)
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Reflects no deduction for fees, expenses or taxes.
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest 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 After-tax returns shown are not relevant to investors who hold shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement plans.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher After-tax returns may exceed the return before taxes due to an assumed tax benefit from realizing a capital loss on a sale of Fund shares.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock
After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of 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 who hold shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement plans. After-tax returns may exceed the return before taxes due to an assumed tax benefit from realizing a capital loss on a sale of Fund shares.
FlexShares Real Assets Allocation Index Fund | Risk Lose Money [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock It is possible to lose money on an investment in the Fund.
FlexShares Real Assets Allocation Index Fund | Risk Not Insured Depository Institution [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.
FlexShares Real Assets Allocation Index Fund | Equity Securities Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Equity Securities Risk is the risk that the values of the equity securities owned by the Fund may be more volatile and underperform other asset classes and the general securities markets.
FlexShares Real Assets Allocation Index Fund | Mid and Small Cap Stock Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Mid and Small Cap Stock Risk is the risk that stocks of mid-sized and smaller companies may be more volatile than stocks of larger, more established companies, and may lack sufficient market liquidity. Mid-sized and small companies may have limited product lines or financial resources, may be dependent upon a particular niche of the market, or may be dependent upon a small or inexperienced management group. Securities of smaller companies may trade less frequently and in lower volume than the securities of larger companies, which could lead to higher transaction costs. Generally the smaller the company size, the greater the risk.
FlexShares Real Assets Allocation Index Fund | Emerging Markets Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Emerging Markets Risk is the risk that emerging markets are generally subject to greater market volatility, political, social and economic instability, uncertain trading markets and more governmental limitations on foreign investments than more developed markets. In addition, companies operating in emerging markets may be subject to lower trading volumes and greater price volatility than companies in more developed markets. Emerging market economies may be based on only a few industries, may be highly vulnerable to changes in local and global trade conditions, and may suffer from extreme and volatile debt burdens or inflation rates. Companies in emerging market countries generally may be subject to less stringent regulatory, disclosure, financial reporting, accounting, auditing and recordkeeping standards than companies in more developed countries. As a result, information, including financial information, about such companies may be less available and reliable, which can impede the Fund’s ability to evaluate such companies. Securities law and the enforcement of systems of taxation in many emerging market countries may change quickly and unpredictably, and the ability to bring and enforce actions (including bankruptcy, confiscatory taxation, expropriation, nationalization of a company’s assets, restrictions on foreign ownership of local
companies, restrictions on withdrawing assets from the country, protectionist measures and practices such as share blocking), or to obtain information needed to pursue or enforce such actions, may be limited. Investments in emerging market securities may be subject to additional transaction costs, delays in settlement procedures, unexpected market closures, and lack of timely information.
FlexShares Real Assets Allocation Index Fund | Foreign Securities Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Foreign Securities Risk is the risk that investing in foreign (non-U.S.) securities may result in the Fund experiencing more rapid and extreme changes in value than a fund that
invests exclusively in securities of U.S. companies, due to less liquid markets and adverse economic, political, diplomatic, financial, and regulatory factors. Foreign governments may impose limitations on foreigners’ ownership of interests in local issuers, restrictions on the ability to repatriate assets, and may also impose taxes. Any of these events could cause the value of the Fund’s investments to decline. Foreign banks, agents and securities depositories that hold the Fund’s foreign assets may be subject to little or no regulatory oversight over, or independent evaluation, of their operations. Additional costs associated with investments in foreign securities may include higher custodial fees than those applicable to domestic custodial arrangements and transaction costs of foreign currency conversions. Unless the Fund has hedged its foreign currency exposure, foreign securities risk also involves the risk of negative foreign currency rate fluctuations, which may cause the value of securities denominated in such foreign currency (or other instruments through which the Fund has exposure to foreign currencies) to decline in value. Currency exchange rates may fluctuate significantly over short periods of time. Currency hedging strategies, if used, are not always successful. For instance, forward foreign currency exchange contracts, if used by the Fund, could reduce performance if there are unanticipated changes in currency exchange rates.
FlexShares Real Assets Allocation Index Fund | Geographic Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Geographic Risk is the risk that if the Fund invests a significant portion of its total assets in certain issuers within the same country or geographic region, an adverse economic, business or political development affecting that country or region may affect the value of the Fund’s investments more, and the Fund’s investments may be more volatile, than if the Fund’s investments were not so concentrated in such country or region.
FlexShares Real Assets Allocation Index Fund | Concentration Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Concentration Risk is the risk that, if the Fund is concentrated in a particular industry or group of industries, the Fund is likely to present more risks than a fund that is broadly diversified over several industries or groups of industries. Compared to the broad market, an individual industry may be more strongly affected by changes in the economic climate, broad market shifts, moves in a particular dominant stock or regulatory changes.
•Real Estate Securities Risk is the risk that the Fund is subject to the risks associated with investment in the real estate sector in addition to the general risk of the stock market. Investing in securities of real estate companies will make the Fund more susceptible to risks associated with the ownership of real estate and with the real estate industry in general, as well as risks that relate specifically to the way in which real estate companies are organized and operated. Real estate companies may have lower trading volumes and may be subject to more abrupt or erratic price movements than the overall securities markets. The value of real estate securities may underperform other sectors of the economy or broader equity markets. To the extent that the Fund concentrates its investments in the real estate sector, it may be subject to greater risk of loss than if it were diversified across different industry sectors.•Natural Resources Sector Risk is the risk that the Fund is subject to the risks associated with investment in companies within the natural resources sector in addition to the general risk of the stock market. Companies within the natural resources sector can be significantly affected by events relating to U.S. and foreign political and economic developments and environmental and other government regulations, as well as other factors including, but not limited to: commodity price volatility, technological developments and natural or man-made disasters. Declines in the demand for, or prices of, natural resources generally would be expected to contribute to declines in the value of the Fund’s or Underlying Fund’s equity securities. Such declines may occur quickly and without warning and may negatively impact the value of the Fund and your investment.•Infrastructure-Related CompaniesRisk is the risk that the Fund is subject to the risks associated with investment in infrastructure-related companies in addition to the general risk of the stock market. Risks associated with infrastructure-related companies include: (a) realized revenue volume may be significantly lower than projected and/or there will be cost overruns; (b) infrastructure project sponsors will alter their terms making a project no longer economical; (c) macroeconomic factors such as low gross domestic product (“GDP”) growth or high nominal interest rates will raise the average cost of infrastructure funding; (d) government regulation may affect rates charged to infrastructure customers; (e) government budgetary constraints will impact infrastructure projects; (f) special tariffs will be imposed; and (g) changes in tax laws, regulatory policies or accounting standards could be unfavorable. Other risks include environmental damage due to a company’s operations or an accident, a natural disaster, changes in market sentiment towards infrastructure and terrorist acts. Any of these events could cause the value of the Fund’s or Underlying Fund’s investments in infrastructure-related companies to decline.
FlexShares Real Assets Allocation Index Fund | Market Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Market Risk is the risk that the value of the Fund’s investments may increase or decrease in response to expected, real or perceived economic, political or financial events in the U.S. or global markets. The frequency and magnitude of such changes in value cannot be predicted. Certain securities and other investments held by the Fund may experience increased volatility, illiquidity, or other potentially adverse effects in response to changing market conditions, inflation, changes in interest rates, lack of liquidity in the bond or equity markets, or volatility in the equity markets. Market disruptions caused by local or regional events such as financial institution failures, war, acts of terrorism, the spread of infectious illness (including epidemics and pandemics) or other public health issues, recessions or other events or adverse investor sentiment could have a significant impact on the Fund and its investments. Such events could result in the Fund’s shares trading at increased premiums or discounts to the Fund’s NAV. During periods of market disruption or other abnormal market conditions, the Fund’s exposure to the risks described elsewhere in this summary will likely increase.
FlexShares Real Assets Allocation Index Fund | Index Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Index Risk is the risk that the Fund would not necessarily buy or sell a security unless that security is added to or removed from, respectively, the Underlying Index, even if that security generally is underperforming, because unlike many investment companies, the Fund does not utilize an investing strategy that seeks returns in excess of the Underlying Index. Additionally, the Fund rebalances and/or reconstitutes its portfolio in accordance with the Underlying Index, and, therefore, any changes to the Underlying Index’s rebalance and/or reconstitution schedule will result in corresponding changes to the Fund’s rebalance and/or reconstitution schedule.
FlexShares Real Assets Allocation Index Fund | Tracking Error Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Tracking Error Risk is the risk that the Fund’s performance may vary from the performance of the Underlying Index as a result of creation and redemption activity, trans
action costs, expenses and other factors. Market disruptions, regulatory restrictions or other abnormal market conditions could have an adverse effect on the Fund's ability to adjust its exposure to required levels in order to track its Index or cause delays in the Underlying Index's rebalancing schedule. During any such delay, it is possible that the Underlying Index, and, in turn, the Fund will deviate from the Underlying Index's stated methodology and therefore experience returns different than those that would have been achieved under a normal rebalancing or reconstitution schedule.
FlexShares Real Assets Allocation Index Fund | Momentum Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Momentum Risk is the risk that securities that have had higher recent price performance compared to other securities may be more volatile than a broad cross-section of securities or that returns on securities that have previously exhibited price momentum are less than returns on other securities or the overall stock market. Momentum can turn quickly and cause significant variation from other types of investments. An Underlying Fund may follow an index that selects constituent securities based on a “momentum” factor.
FlexShares Real Assets Allocation Index Fund | Value Investing Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Value Investing Risk is the risk that an Underlying Fund’s investment in companies whose securities are believed to be undervalued, relative to their underlying profitability, will not appreciate in value as anticipated.
FlexShares Real Assets Allocation Index Fund | Quality Factor Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Quality Factor Risk is the risk that the past performance of companies that have exhibited quality characteristics does not continue or the returns on securities issued by such companies may be less than returns from other styles of investing or the overall stock market. There may be periods when quality investing is out of favor and during which time the Fund's performance may suffer.
FlexShares Real Assets Allocation Index Fund | Authorized Participant Concentration Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Authorized Participant Concentration Risk is the risk that the Fund may be adversely affected because it has a limited number of institutions that act as authorized participants (“Authorized Participants”). Only an Authorized Participant may engage in creation or redemption transactions directly with the Fund and none of those Authorized Participants is obligated to engage in creation and/or redemption transactions. To the extent that these institutions exit the business or are unable or unwilling to proceed with creation and/or redemption orders with respect to the Fund and no other Authorized Participant is able or willing to step forward to create or redeem Creation Units (as defined below), Fund shares may trade at a discount to NAV and possibly face trading halts and/or delisting.
FlexShares Real Assets Allocation Index Fund | Calculation Methodology Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Calculation Methodology Risk is the risk that the Underlying Index’s calculation methodology or sources of information may not provide an accurate assessment of included issuers or correct valuation of securities, nor is the availability or timeliness of the production of the Underlying Index guaranteed. A security included in the Underlying Index may not exhibit the characteristic or provide the specific exposure for which it was selected and consequently a Fund's holdings may not exhibit returns consistent with that characteristic or exposure.
FlexShares Real Assets Allocation Index Fund | Market Trading Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Market Trading Risk is the risk that the Fund and the Underlying Funds face because their shares are listed on a securities exchange, including the potential lack of an active market for Fund shares, losses from trading in secondary markets, periods of high volatility and disruption in the creation/redemption process of the Fund or Underlying Fund. ANY OF THESE FACTORS MAY LEAD TO THE FUND’S SHARES TRADING AT A PREMIUM OR DISCOUNT TO NAV.
Trading in Fund shares may be halted due to market conditions or for reasons that, in the view of its listing exchange, make trading in the shares inadvisable. The market prices of Fund shares will generally fluctuate in accordance with
changes in its NAV, changes in the relative supply of, and demand for, Fund shares, and changes in the liquidity, or the perceived liquidity, of the Fund’s holdings.
FlexShares Real Assets Allocation Index Fund | Derivatives Risks [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Derivatives Risk is the risk that derivatives may pose risks in addition to and greater than those associated with investing directly in securities, currencies and other instruments, may be illiquid or less liquid, more volatile, more difficult to value and leveraged so that small changes in the value of the underlying instrument may produce disproportionate losses to the Fund. Derivatives are also subject to counterparty risk, which is the risk that the other party to the transaction will not perform its contractual obligations. The use of derivatives is a highly specialized activity that involves investment techniques and risks different from those associated with investments in more traditional securities and instruments.
•Futures Contracts Risk is the risk that there will be imperfect correlation between the change in market value of the Fund’s securities and the price of futures contracts, which may result in the strategy not working as intended; the possible inability of the Fund to sell or close out a futures contract at the desired time or price; losses due to unanticipated market movements, which potentially are unlimited; and the possible inability of NTI to correctly predict the direction of securities’ prices, interest rates, currency exchange rates and other economic factors, which may make the Fund’s returns more volatile or increase the risk of loss.•Options Contracts Risk Options contracts give the holder of the option the right to buy (or to sell) a position in a security or in a contract to the writer of the option, at a certain price. They are subject to correlation risk because there may be an imperfect correlation between the options and the securities markets that cause a given transaction to fail to achieve its objectives. The successful use of options depends on the investment adviser’s ability to predict correctly future price fluctuations and the degree of correlation between the options and securities markets. Exchanges can limit the number of positions that can be held or controlled by the Fund or the investment adviser, thus limiting the ability to implement the Fund’s strategies.•Forward Foreign Currency Contracts Risk is the risk that, if forward prices increase, a loss will occur to the extent that the agreed upon purchase price of the currency exceeds the price of the currency that was agreed to be sold.
FlexShares Real Assets Allocation Index Fund | Securities Lending Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Securities Lending Risk is the risk that the Fund may lose money because the borrower of the loaned securities fails to return the securities in a timely manner or at all. The Fund could also lose money in the event of a decline in the value of collateral provided for loaned securities or a decline in the value of any investments made with cash collateral.
FlexShares Real Assets Allocation Index Fund | Valuation Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Valuation Risk is the risk that the sale price the Fund or an Underlying Fund could receive for a portfolio security may differ from the Fund’s or the Underlying Fund’s valuation
of the security, particularly for securities that trade in low volume or volatile markets or that are valued using a fair value methodology. In addition, the value of the securities in the Fund’s or an Underlying Fund’s portfolio may change on days when shareholders will not be able to purchase or sell the Fund’s shares.
FlexShares Real Assets Allocation Index Fund | MLP Tax Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
MLP Tax Risk. An MLP is a public limited partnership or limited liability company taxed as a partnership under the Internal Revenue Code of 1986. MLPs taxed as partnerships do not pay U.S. federal income tax at the partnership level, subject to the application of certain partnership audit rules. A change in current tax law, or a change in the underlying business mix of a given MLP, however, could result in an MLP being classified as a corporation for U.S. federal income tax purposes, which would have the effect of reducing the amount of cash available for distribution by the MLP and, as a result, could result in a reduction of the value of the Fund’s investment (and the Fund), and, consequently, lower income.
FlexShares Real Assets Allocation Index Fund | REIT Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
REIT Risk is the risk that the Fund’s investments may be affected by factors affecting REITs and the real estate sector generally. Investing in REITs involves certain unique risks in addition to those risks associated with investing in the real estate industry in general. REITs whose underlying properties are concentrated in a particular industry or geographic region are also subject to risks affecting such industries and regions. REITs may have limited financial resources, may trade less frequently and in lower volume, engage in dilutive offerings or become more volatile than other securities. By investing in REITs through the Fund, a shareholder will bear expenses of the REITs in addition to expenses of the Fund. In addition, REITs could possibly fail to (i) qualify for favorable tax treatment under applicable tax law, or (ii) maintain their exemption from registration under the Investment Company Act of 1940.
FlexShares Real Assets Allocation Index Fund | MLP Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
MLP Risk is the risk that accompanies an investment in MLP units. The risks of investing in an MLP are similar to those of investing in a partnership, including more flexible governance structures, which could result in less protection for investors, such as the Fund, than investments in a corporation. An investor in an MLP normally would not be liable for the debts of the MLP beyond the amount that the investor has contributed but may not be shielded from liability to the same extent that a shareholder of a corporation would be. MLPs are also subject to risks related to potential conflicts of interest between the MLP and the MLP’s general partner and cash flow risks. MLPs that concentrate in a particular industry or a particular geographic region are subject to risks associated with such industry or region. MLPs may also be sensitive to changes in interest rates and during periods of interest rate volatility, limited capital markets access and/or low commodities pricing and may not provide attractive returns.
FlexShares Real Assets Allocation Index Fund | Interest Rate Risks [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Interest Rate Risk is the risk that during periods of rising interest rates, the Fund’s yield (and the market value of its securities) will tend to be lower than prevailing market
rates; in periods of falling interest rates, the Fund’s yield (and the market value of its securities) will tend to be higher. Increases in interest rates typically lower the present value of a REIT’s future earnings stream, and may make financing property purchases and improvements more costly. Because the market price of REIT stocks may change based upon investors’ collective perceptions of future earnings, the value of the Fund may decline when investors anticipate or experience rising interest rates.
In general, securities with longer maturities or durations are more sensitive to interest rate changes. Changing interest rates, including rates that fall below zero, may have unpredictable effects on the markets and the Fund’s investments, may result in heightened market volatility, may impact the liquidity of fixed-income securities and of the Fund, and may detract from Fund performance. A low or negative interest rate environment could cause the Fund’s earnings to fall below the Fund’s expense ratio, resulting in a low or negative yield and a decline in the Fund’s NAV. A general rise in interest rates may cause investors to move out of fixed income securities on a large scale, which could adversely affect the price and liquidity of fixed income securities and could also result in increased redemptions for the Fund. During periods when inflation rates are high or rising, or during periods of low interest rates, the Fund may be subject to a greater risk of rising interest rates.
FlexShares Real Assets Allocation Index Fund | Real Estate Securities Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Real Estate Securities Risk is the risk that the Fund or an Underlying Fund is subject to the risks associated with investment in the real estate sector in addition to the general risk of the stock market. Investing in securities of real estate companies will make an Underlying Fund, and the
Fund, more susceptible to risks associated with the ownership of real estate and with the real estate industry in general, as well as risks that relate specifically to the way in which real estate companies are organized and operated. Real estate companies may have lower trading volumes and may be subject to more abrupt or erratic price movements than the overall securities markets. The value of real estate securities may underperform other sectors of the economy or broader equity markets. To the extent that an Underlying Fund, and the Fund, concentrates its investments in the real estate sector, it may be subject to greater risk of loss than if it were diversified across different industry sectors.
FlexShares Real Assets Allocation Index Fund | Tax Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Tax Risk is the risk that, because the Fund invests in the Underlying Funds, the Fund’s realized losses on sales of shares of the Underlying Funds may be indefinitely or permanently deferred as “wash sales.” Distributions of short-term capital gains by the Underlying Funds will be recognized as ordinary income by the Fund and would not be offset by the Fund’s capital loss carryforwards, if any. Capital loss carryforwards of the Underlying Funds, if any, would not offset net capital gains of the Fund. Each of these effects is caused by the Fund’s investment in the Underlying Funds and may result in distributions to Fund shareholders being of higher magnitudes and less likely to qualify for lower capital gain tax rates than if the Fund were to invest otherwise.
FlexShares Real Assets Allocation Index Fund | Model Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Model Risk is the risk that the asset allocation model used to calculate the Underlying Index will not achieve its intended results.
FlexShares Real Assets Allocation Index Fund | Fund of Funds Risk [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock
Fund of Funds Risk is the risk that the Fund’s investment performance largely depends on the investment performance of the Underlying Funds in which it primarily invests. The Fund’s NAV will change with changes in the value of the Underlying Funds and other securities in which the Fund invests based on their market valuations. An investment of the Fund is subject to the risks associated with the Underlying Funds that comprise the Underlying Index. There is the risk that the Index Provider’s evaluations and assumptions regarding the asset classes represented by the Underlying Funds in the Underlying Index at any given time may be incorrect based on actual market conditions.
An investment in the Fund will entail more costs and expenses than direct investments in the Underlying Funds. The Fund will indirectly pay a proportional share of the
expenses of the Underlying Funds in which it invests (including operating expenses and management fees), in addition to the fees and expenses it pays directly. As the Fund’s allocation to the Underlying Funds changes from time to time, or to the extent that the expense ratio of the Underlying Funds changes, the weighted average operating expenses borne by the Fund may increase or decrease.
FlexShares Real Assets Allocation Index Fund | FlexShares Real Assets Allocation Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.49%
Distribution (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.01%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.48%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.98% [1]
Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.47%) [2]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.51%
1 Year rr_ExpenseExampleYear01 $ 52
3 Years rr_ExpenseExampleYear03 265
5 Years rr_ExpenseExampleYear05 496
10 Years rr_ExpenseExampleYear10 $ 1,159
2016 rr_AnnualReturn2016 8.21%
2017 rr_AnnualReturn2017 14.92%
2018 rr_AnnualReturn2018 (8.40%)
2019 rr_AnnualReturn2019 23.59%
2020 rr_AnnualReturn2020 (0.90%)
2021 rr_AnnualReturn2021 18.68%
2022 rr_AnnualReturn2022 (10.04%)
2023 rr_AnnualReturn2023 5.51%
2024 rr_AnnualReturn2024 0.66%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 14.27%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (23.50%)
One Year rr_AverageAnnualReturnYear01 0.66%
Five Year rr_AverageAnnualReturnYear05 2.36%
Since Inception of Fund rr_AverageAnnualReturnSinceInception 4.98%
Inception Date of Fund rr_AverageAnnualReturnInceptionDate Nov. 23, 2015
FlexShares Real Assets Allocation Index Fund | After Taxes on Distributions | FlexShares Real Assets Allocation Index Fund  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 (0.36%)
Five Year rr_AverageAnnualReturnYear05 1.58%
Since Inception of Fund rr_AverageAnnualReturnSinceInception 4.10%
FlexShares Real Assets Allocation Index Fund | After Taxes on Distributions and Sale of Shares | FlexShares Real Assets Allocation Index Fund  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 0.85%
Five Year rr_AverageAnnualReturnYear05 1.70%
Since Inception of Fund rr_AverageAnnualReturnSinceInception 3.72%
FlexShares Real Assets Allocation Index Fund | MSCI ACWI Index  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 17.49% [3]
Five Year rr_AverageAnnualReturnYear05 10.06% [3]
Since Inception of Fund rr_AverageAnnualReturnSinceInception 10.20% [3]
FlexShares Real Assets Allocation Index Fund | Northern Trust Real Assets Allocation Index℠  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 0.72% [3]
Five Year rr_AverageAnnualReturnYear05 2.47% [3]
Since Inception of Fund rr_AverageAnnualReturnSinceInception 5.10% [3]
[1] The “Total Annual Fund Operating Expenses” may not correlate to the ratio of expenses to average net assets as reported in the "Financial Highlights" section of the Prospectus, which reflects the operating expenses of the Fund and does not include “Acquired Fund Fees and Expenses.”
[2] Northern Trust Investments, Inc. (“NTI”) has contractually agreed to reimburse a portion of the operating expenses of the Fund (other than 12b-1 Fees, Tax Expenses, Extraordinary Expenses, and Acquired Fund Fees and Expenses (“Acquired Fund Fees and Expenses”)) to the extent the “Total Annual Fund Operating Expenses” exceed 0.49%. This contractual limitation may not be terminated before March 1, 2026 without the approval of the Fund’s Board of Trustees. NTI also has contractually agreed until March 1, 2026 to waive a portion of its Management Fees and/or reimburse certain operating expenses in an amount equal to the Acquired Fund Fees and Expenses attributable to the Fund’s investments in the Underlying Fund (as defined below). The Fund’s Board of Trustees may terminate the contractual agreements at any time if it determines that it is in the best interest of the Fund and its shareholders.
[3] Reflects no deduction for fees, expenses or taxes.