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FlexShares Core Select Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading FlexShares® Core Select Bond Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
The Fund seeks total return and preservation of capital.
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 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 each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination March 1, 2023
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 50% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 50.00%
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 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 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 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 seeks to achieve its investment objective by investing, under normal circumstances, at least 80% of its net assets in U.S. dollar-denominated investment-grade fixed-income securities either directly or indirectly through exchange-traded funds ("ETFs") and other registered investment companies (together with ETFs, “Underlying Funds”). The Fund primarily invests in fixed-rate securities of varying maturities, including bonds, debt securities and other similar instruments issued by U.S. public and U.S and non-U.S. private sector entities. Such issuers include, without limitation, U.S. governments and their agencies, instrumentalities or sponsored enterprises, and U.S. and non-U.S. private-sector entities, such as corporations and banks. The Fund may invest in fixed income securities of any maturity. 
The Fund seeks to provide attractive risk-adjusted performance by investing in a portfolio of fixed-income securities and Underlying Funds. The Fund generally adjusts its allocations to securities based on NTI’s assessment of potential changes in interest rate levels, the shape of the yield curve and credit spread relationships. The Fund also seeks to balance the potential for return and risk while emphasizing liquidity and diversification across a spectrum of U.S. dollar-denominated investment-grade fixed income securities. 
The Fund may invest, without limitation, in mortgage- or asset-backed securities, including to-be-announced (“TBA”) transactions, and purchase and sell securities on a when-issued, delayed delivery or forward commitment basis. The Fund also may, without limitation, seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts (such as buy backs or mortgage dollar rolls). 
When necessary or appropriate to manage the Fund’s exposure to interest-rate risks, the Fund may take short or long positions in U.S. Treasury futures or transact in interest rate swaps. The Fund’s short positions in U.S. Treasury futures are not intended to mitigate credit spread risk or other factors influencing the price of non-government bonds, which may have a greater impact than interest rates. The Fund may also invest in other interest rate futures contracts, including, but not limited to, Eurodollar and Federal Funds futures. 
The Fund may invest in short-term paper, cash and cash equivalents, including shares of affiliated and unaffiliated money market funds, in order to achieve its investment objective. 
The Fund may make the foregoing investments either directly or indirectly by investing in Underlying Funds, such as indexed or actively managed ETFs or other registered investment companies that invest in such securities. The Underlying Funds in which the Fund may invest include funds managed by NTI or its affiliates. 
The Fund is an actively managed ETF that does not seek to replicate the performance of a specified index. The Fund may have a higher degree of portfolio turnover than funds that seek to replicate the performance of an 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). 
Risk [Heading] rr_RiskHeading Principal Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
As with any investment, you could lose all or part of your investment in the Fund, and the Fund’s performance could trail that of other investments. The Fund is subject to certain risks, including the principal risks noted below, any of which may adversely affect the Fund’s net asset value (“NAV”), trading price, yield, total return and ability to meet its investment objective. Because the Fund may significantly invest in Underlying Funds, the risks described below are also applicable to the Underlying Funds, and to the extent that the Fund invests directly in securities and other instruments, the risks described below are also directly applicable to the Fund. Each risk noted below is considered a principal risk of investing in the Fund, regardless of the order in which it appears. The significance of each risk factor below may change over time and you should review each risk factor carefully.
 
Management Risk is the risk that a strategy used by NTI may fail to produce the intended results or that imperfections, errors or limitations in the tools and data used by NTI may cause unintended results. 
Underlying Fund Risk is the risk that the Fund’s investment performance and its ability to achieve its investment objective may be directly related to the performance of the Underlying Funds in which it invests. There can be no assurance that the Underlying Funds will achieve their respective investment objectives. The Fund is subject to the risks of the Underlying Funds in direct proportion to the allocation of its assets among the Underlying Funds. By investing in the Underlying Funds indirectly through the Fund, you will incur not only a proportionate share of the expenses of the Underlying Fund held by the Fund (including operating costs and investment management fees), but also expenses of the Fund. 
Sampling Risk is the risk that the index-based ETFs in which the Fund invests may utilize a representative sampling approach to track their respective underlying indices. ETFs that utilize a representative sampling approach are subject to an increased risk of tracking error because the securities selected for the ETF in the aggregate may vary from the investment profile of the underlying index. Additionally, if using a representative sampling approach, an ETF will typically hold a smaller number of securities than the underlying index, and as a result, an adverse develop- 
ment to an issuer of securities that the ETF holds could result in a greater decline in NAV than would be the case if the ETF held all of the securities in the underlying index. 
Tracking Risk is the risk that the index-based ETFs and other investment companies in which the Fund invests may not be able to replicate exactly the performance of the indices they track as a result of share purchases and redemptions, transaction costs, expenses and other factors. In addition, index-based ETFs and other investment companies in which the Fund invests may incur expenses not incurred by their applicable indices. Certain securities comprising the indices may, from time to time, temporarily be unavailable, which may further impede an ETF’s or other investment company’s ability to track its applicable index or match its performance. 
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 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 and could result in the Fund’s shares trading at increased premiums or discounts to the Fund’s NAV. An outbreak of COVID-19, a respiratory disease caused by a novel coronavirus, has negatively affected the worldwide economy, the financial health of individual companies and the market in significant and unforeseen ways. The future impact of COVID-19 is currently unknown. The effects to public health, business and market conditions resulting from the COVID-19 pandemic may have a significant negative impact on the performance of the Fund’s investments. 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. 
Market risk includes the risk that a particular style of investing, such as growth or value, may underperform the market generally. The market value of the securities in 
which the Fund invests may go up or down in response to the prospects of individual companies, particular sectors or governments and/or general economic conditions throughout the world due to increasingly interconnected global economies and financial markets. As a result, whether or not the Fund invests in securities of issuers located in or with significant exposure to the countries directly affected, the value and liquidity of the Fund’s investments may be negatively affected. Securities markets may experience great short-term volatility and may fall sharply at times. Different markets may behave differently from each other and a foreign market may move in the opposite direction from the U.S. market. Price changes may be temporary or last for extended periods. You could lose money over short periods due to fluctuation in the market prices of the Fund’s shares in response to market movements, and over longer periods during market downturns. 
Income Risk is the risk that the Fund’s income may decline when interest rates fall. This decline can occur because the Fund must invest in lower-yielding bonds as bonds in its portfolio mature, bonds in an Underlying Fund’s underlying index are substituted or the Fund otherwise needs to purchase additional bonds. 
Asset-Backed and Mortgage-Backed Securities Risk. Asset-backed and mortgage-backed securities may be less liquid than other bonds, and may be more sensitive than other bonds to the market’s perception of issuers and creditworthiness of payees, particularly in declining general economic conditions when concern regarding mortgagees’ ability to pay (e.g., the ability of homeowners, commercial mortgagees, consumers with student loans, automobile loans or credit card debtholders to make payments on the underlying loan pools) rises, which may result in the Fund experiencing difficulty selling or valuing these securities. In addition, these securities may not be backed by the full faith and credit of the U.S. government, have experienced extraordinary weakness and volatility at various times in recent years, and may decline quickly in the event of a substantial economic or market downturn. Those asset-backed and mortgage-backed securities that are guaranteed as to the timely payment of interest and principal by a government entity, are not guaranteed as to market price, which will fluctuate. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain asset-backed and mortgage-backed securities. 
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. 
Market Trading Risk is the risk that the Fund faces because its 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. 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 price 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 perceived liquidity, of the Fund’s holdings. 
Credit (or Default) Risk is the risk that the inability or unwillingness of an issuer or guarantor of a fixed-income security, or a counterparty to a TBA, repurchase or other transaction, to meet its payment or other financial obligations will adversely affect the value of the Fund’s investments and its returns. The credit quality of a debt security or of the issuer of a debt security held by the Fund could deteriorate rapidly, which may impair the Fund's liquidity or cause a deterioration in the Fund's NAV. The Fund could also be delayed or hindered in its enforcement of rights against an issuer, guarantor or counterparty. The degree of credit risk depends on the issuer’s or counterparty’s financial condition and on the terms of the securities. 
Debt Extension Risk is the risk that when interest rates rise an issuer will exercise its right to pay principal on certain debt securities held by the Fund later than expected. This 
will cause the value of the security, and the Fund’s NAV, to decrease, and the Fund may lose opportunities to invest in higher yielding securities. 
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.
• Swap Contracts Risk Swap contracts involve the risk that the counterparty with whom the Fund has entered into the agreement will default on its obligation to pay the Fund. While certain swaps are subject to mandatory central clearing, which is intended to reduce counterparty risk, these swaps are subject to the risk that a central clearinghouse will go into bankruptcy or become non-operational, and sometimes involve increased transaction costs. 
Sector Risk is the risk that companies in similar businesses may be similarly affected by particular economic or market events, which may, in certain circumstances, cause the value of securities of all companies in a particular sector of the market to decrease. 
Hedging Risk is the risk that the Fund’s short positions in U.S. Treasury futures and transactions in interest rate swaps will not completely eliminate interest rate risk of 
long positions in bonds. The Fund seeks to mitigate the potential impact of interest rates on the performance of bonds by entering into short positions in U.S. Treasury futures or transact in interest rate swaps. The Fund’s short positions in U.S. Treasury futures and interest rate swaps are not intended to mitigate credit spread risk or other factors influencing the price of bonds, which may have a greater impact than interest rates. In addition, when interest rates fall, long-only bond investments will perform better than the Fund’s investments. In certain falling interest rate environments, the Fund’s hedging strategy could result in disproportionately larger losses in the short U.S. Treasury futures and interest rate swaps positions as compared to gains in the long bond positions attributable to interest rate changes. There is no guarantee the Fund will have positive returns, even in environments of sharply rising Treasury interest rates in which the Fund’s short positions might be expected to mitigate the effects of such rises. The Fund will incur expenses when entering into short positions. 
Interest Rate/Maturity Risk is the risk that the value of the Fund’s fixed-income assets will decline because of rising interest rates. In general, securities with longer maturities or durations are more sensitive to interest rate changes. Risks associated with rising interest rates are heightened given that interest rates in the U.S. are currently at historically low levels. 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 negative yield and a decline in the Fund’s share price. An increase in interest rates may cause investors to move out of fixed income securities on a large scale, which could adversely affect the price of fixed income securities, lead to heightened volatility in the fixed-income markets and may adversely affect the liquidity of certain fixed-income investments. 
Liquidity Risk is the risk that certain portfolio securities may be less liquid or may become less liquid than others, which may make them difficult or impossible to sell at the time and the price that the Fund would like, adversely affecting the value of the Fund’s investments and its 
returns. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. Liquidity risk may result from the lack of an active market, reduced number and capacity of traditional market participants to make a market in fixed income securities, and may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed income funds may be higher than normal, causing increased supply in the market due to selling activity. The market for certain investments may become illiquid under adverse market or economic conditions independent of any specific adverse changes in the conditions of a particular issuer. Additionally, in adverse market conditions, the Fund’s market price may begin to reflect illiquidity or pricing uncertainty of the Fund’s portfolio securities. This could lead to the Fund’s shares trading at a price that is higher or lower than the Fund’s NAV. At times, such differences may be significant. 
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. 
Prepayment (or Call) Risk is the risk that an issuer could exercise its right to pay principal on callable debt securities held by the Fund earlier than expected. Issuers may be more likely to prepay when interest rates fall, when credit spreads change, or when an issuer’s credit quality improves. If this happens, the Fund will not benefit from the rise in the market price of the securities that normally accompanies a decline in interest rates, and will be forced to reinvest prepayment proceeds in lower yielding securities, which may reduce the Fund’s returns. The Fund may also lose any premium it paid to purchase the securities. 
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. 
U.S. Government Securities Risk is the risk that the U.S. government will not provide financial support to its agencies, instrumentalities or sponsored enterprises if it is not obligated to do so by law. Many U.S. government securities that may be purchased by the Fund are not backed by the full faith and credit of the United States. The maximum potential liability of the issuers of some U.S. government securities may greatly exceed their current resources, including any legal right to support from the U.S. Treasury. It is possible that the issuers of such securities will not have the funds to meet their payment obligations in the future. 
It is possible to lose money on an investment in the Fund. 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. 
Risk Lose Money [Text] rr_RiskLoseMoney It is possible to lose money on an investment in the Fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution 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. 
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. 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.
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 (6/30/2020): 4.28%
Worst Quarter (3/31/2021): -3.48%
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for the periods ended December 31, 2021)
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 Core Select Bond Fund | FlexShares Core Select Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
Management Fees rr_ManagementFeesOverAssets 0.35%
Distribution (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.01%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.18% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.54%
Expense Reimbursement rr_FeeWaiverOrReimbursementOverAssets (0.19%) [2]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.35%
1 Year rr_ExpenseExampleYear01 $ 36
3 Years rr_ExpenseExampleYear03 154
5 Years rr_ExpenseExampleYear05 283
10 Years rr_ExpenseExampleYear10 $ 659
2017 rr_AnnualReturn2017 3.87%
2018 rr_AnnualReturn2018 (1.27%)
2019 rr_AnnualReturn2019 9.29%
2020 rr_AnnualReturn2020 8.75%
2021 rr_AnnualReturn2021 (1.82%)
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Best Quarter
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 4.28%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Worst Quarter
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2021
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (3.48%)
One Year rr_AverageAnnualReturnYear01 (1.82%)
Five Year rr_AverageAnnualReturnYear05 3.65%
Since Inception of Fund rr_AverageAnnualReturnSinceInception 3.44%
Inception Date of Fund rr_AverageAnnualReturnInceptionDate Nov. 18, 2016
FlexShares Core Select Bond Fund | After Taxes on Distributions | FlexShares Core Select Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 (2.50%)
Five Year rr_AverageAnnualReturnYear05 2.58%
Since Inception of Fund rr_AverageAnnualReturnSinceInception 2.33%
FlexShares Core Select Bond Fund | After Taxes on Distributions and Sale of Shares | FlexShares Core Select Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 (1.08%)
Five Year rr_AverageAnnualReturnYear05 2.33%
Since Inception of Fund rr_AverageAnnualReturnSinceInception 2.15%
FlexShares Core Select Bond Fund | Bloomberg U.S. Aggregate Bond Index  
Risk/Return: rr_RiskReturnAbstract  
One Year rr_AverageAnnualReturnYear01 (1.54%) [3]
Five Year rr_AverageAnnualReturnYear05 3.57% [3]
Since Inception of Fund rr_AverageAnnualReturnSinceInception 3.51% [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 include "Acquired Fund Fees and Expenses."
[2] Northern Trust Investments, Inc. (“NTI” or the “Investment Adviser”) has contractually agreed to reimburse a portion of the operating expenses of the Fund (other than Acquired Fund Fees and Expenses) to the extent the “Total Annual Fund Operating Expenses” exceed 0.35%. This contractual limitation may not be terminated before March 1, 2023 without the approval of the Fund’s Board of Trustees. NTI has also contractually agreed to waive Management Fees or reimburse certain expenses in an amount equal to the sum of (a) any Acquired Fund Fees and Expenses, if any, incurred by the Fund that are attributable to the Fund’s investment in Acquired Funds managed by NTI or an investment adviser controlling, controlled by, or under common control with NTI (“Affiliated Funds”); and (b) 0.05% or such lesser amount in Acquired Fund Fees and Expenses incurred by the Fund that are attributable to the Fund’s investment in Acquired Funds that are not Affiliated Funds, until March 1, 2023. The Trust’s Board of Trustees may terminate the contractual arrangements 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.