Objective, Primary [Text Block]

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Risk/Return [Heading]

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FlexShares® US Quality Low Volatility Index Fund

Objective [Heading]

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Investment Objective

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The Fund seeks investment results that correspond generally to the price and yield performance, before fees and expenses, of the Northern Trust Quality Low Volatility Index^SM (the “Underlying Index”).

Expense [Heading]

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Fees and Expenses of the Fund

Expense Narrative [Text Block]

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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]

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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

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March 1, 2023

Portfolio Turnover [Heading]

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Portfolio Turnover.

Portfolio Turnover [Text Block]

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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 43% of the average value of its portfolio.

Portfolio Turnover, Rate

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43.00%

Expense Example [Heading]

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Example

Expense Example Narrative [Text Block]

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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]

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Principal Investment Strategies

Strategy Narrative [Text Block]

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The Underlying Index is designed to reflect the performance of a selection of companies that, in aggregate, possess lower overall absolute volatility characteristics relative to the Northern Trust 1250 Index (the “Parent Index”), a float-adjusted market capitalization weighted index of U.S. domiciled large- and mid-capitalization companies. In addition, the Underlying Index is designed to select companies from the Parent Index that exhibit financial strength, stability and enhanced risk-return characteristics, which Northern Trust Investments, Inc. (“NTI” or the “Investment Adviser”) believes can provide equity-market participation while seeking to protect against downside risks during certain market environments.

To derive the Underlying Index, NTI, acting in its capacity as the index provider (the “Index Provider”) ranks all constituents of the Parent Index using a Northern Trust proprietary quality factor. This factor is a quantitative ranking based on: (a) management expertise (e.g. corporate finance activities); (b) profitability (e.g. reliability and sustainability of financial performance); and (c) cash flow (e.g., cash flow generation). The Index Provider then excludes the lowest quintile of constituents ranked accord-

ing to the quality factor and uses an optimization process to select and weight eligible securities in order to (a) seek to reduce overall portfolio volatility and (b) maximize the overall quality factor relative to the Parent Index. The optimization also includes sector, industry group, single-security weight, historical beta (i.e. market sensitivity, relative to the Parent Index) and turnover constraints so that these characteristics of the Underlying Index vary within acceptable bands relative to the Parent Index.

As of December 31, 2021, the Underlying Index was comprised of 110 constituent securities with market capitalizations ranging from $3.9 billion to $2.9 trillion. The Underlying Index is governed by published, objective rules for security selection, exclusion, weighting, rebalancing and adjustments for corporate actions. It is reconstituted quarterly. The Fund generally reconstitutes 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 uses a representative sampling strategy to manage the Fund. “Representative sampling” is investing in a representative sample of securities that collectively has an investment profile similar to the Underlying Index. The Fund may or may not hold all of the securities that are included in the Underlying Index. The Fund reserves the right to invest in substantially all of the securities in its Underlying Index in approximately the same proportions (i.e., replication) if NTI determines that it is in the best interest of the Fund.

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, as the Index Provider. NTI also serves as the investment adviser to the Fund. 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).

The Fund is “non-diversified” under the Investment Company Act of 1940 (“1940 Act”), as amended, and may invest more of its assets in fewer issuers than “diversified” funds.

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.

Strategy Portfolio Concentration [Text]

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Risk [Heading]

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Principal Risks

Risk Narrative [Text Block]

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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. 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.

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.

Low Volatility Risk is the risk that although the Underlying Index is designed to have overall volatility that is lower than that of the Parent Index, there is no guarantee it will be successful. Securities or other assets in the Fund’s portfolio may be subject to price volatility and the prices may not be any less volatile than the market as a whole and could be more volatile. The market prices of the securities or other assets in the Fund’s portfolio may fluctuate, sometimes rapidly and unpredictably. The Fund will continue to seek to track the Underlying Index even if the Underlying Index does not have lower overall volatility than the Parent Index. There is also the risk that the Fund may experience volatility greater than that of the Parent Index as a result of

tracking error. A portfolio of securities with greater volatility is generally considered to have a higher risk profile than a portfolio with lower volatility.

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.

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.

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 disruptions, regulatory restrictions or other abnormal market conditions could have an adverse effect on the Fund’s ability to adjust its exposure to the required levels in order to track its Underlying Index or cause delays in the Underlying Index’s rebalancing or reconstitution 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. Because the Fund is designed to maintain a high level of exposure to its Underlying Index at all times, it will not take any steps to invest defensively or otherwise reduce the risk of loss during market downturns.

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.

Index Risk is the risk that the Fund would not necessarily buy or sell a security unless that security is added or removed, respectively, from 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 an 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.

Tracking Error Risk is the risk that the Fund’s performance may vary substantially from the performance of the Underlying Index. The Fund’s performance may vary from the performance of the Underlying Index for a number of reasons including that the Fund incurs operating expenses that the Underlying Index does not and that the Fund accepts custom baskets.

Sampling Risk is the risk that the Fund’s use of a representative sampling approach may result in increased tracking error because the securities selected for the Fund in the aggregate may vary from the investment profile of the Underlying Index. Additionally, the use of a representative sampling approach may result in the Fund holding a smaller number of securities than the Underlying Index, and, as a result, an adverse development to an issuer of securities that the Fund holds could result in a greater decline in NAV than would be the case if the Fund held all of the securities in the Underlying Index.

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.

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 Index guaranteed. A security included in an 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. Unusual market conditions may cause the provider of the Underlying Index to postpone a scheduled rebalance, which could cause the Underlying Index to vary from its normal or expected composition.

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 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.

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.

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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.

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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.

Mid Cap Stock Risk is the risk that stocks of mid-sized companies may be subject to more abrupt or erratic market movements than stocks of larger, more established companies, and may lack sufficient market liquidity. Mid-sized companies may have limited product lines or financial resources, and 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.

Non-Diversification Risk is the risk that because the Fund is non-diversified and may invest a larger percentage of its assets in the securities of fewer issuers than a diversified fund, the Fund’s performance will be more vulnerable to changes in the market value of a single issuer or group of issuers, and more susceptible to risks associated with a single economic, political or regulatory occurrence.

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.

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]

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It is possible to lose money on an investment in the Fund.

Risk Not Insured Depository Institution [Text]

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An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal DepositInsurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.

Risk Nondiversified Status [Text]

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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 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]

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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]

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flexshares.com

Performance Past Does Not Indicate Future [Text]

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Past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future.

Bar Chart [Heading]

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Calendar Year Total Returns

Bar Chart Closing [Text Block]

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For the period shown in the bar chart above:

Best Quarter (6/30/2020): 15.80%

Worst Quarter (3/31/2020): -18.55%

Performance Table Heading

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Average Annual Total Returns (for the periods ended December 31, 2021)

Index No Deduction for Fees, Expenses, Taxes [Text]

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Reflects no deduction for fees, expenses or taxes.

Performance Table Uses Highest Federal Rate

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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

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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

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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

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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 dif-

fer 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 US Quality Low Volatility Index Fund | FlexShares US Quality Low Volatility Index Fund

Distribution (12b-1) Fees

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Management Fees

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0.22%

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none

Other Expenses

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0.01%

Total Annual Fund Operating Expenses

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0.23%

Expense Reimbursement

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(0.01%)

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Total Annual Fund Operating Expenses After Expense Reimbursement

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0.22%

1 Year

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$ 23

3 Years

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5 Years

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129

10 Years

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$ 292

2020

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9.45%

2021

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26.07%

Highest Quarterly Return, Label

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Best Quarter

Highest Quarterly Return, Date

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Jun. 30, 2020

Highest Quarterly Return

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15.80%

Lowest Quarterly Return, Label

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Worst Quarter

Lowest Quarterly Return, Date

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Mar. 31, 2020

Lowest Quarterly Return

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(18.55%)

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26.07%

FlexShares US Quality Low Volatility Index Fund | After Taxes on Distributions | FlexShares US Quality Low Volatility Index Fund

rr_AverageAnnualReturnSinceInception

16.74%

Inception Date of Fund

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Jul. 15, 2019

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rr_AverageAnnualReturnYear01

25.75%

FlexShares US Quality Low Volatility Index Fund | After Taxes on Distributions and Sale of Shares | FlexShares US Quality Low Volatility Index Fund

rr_AverageAnnualReturnSinceInception

16.38%

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rr_AverageAnnualReturnYear01

15.62%

FlexShares US Quality Low Volatility Index Fund | Russell 1000® Index

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13.00%

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rr_AverageAnnualReturnYear01

26.46%

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FlexShares US Quality Low Volatility Index Fund | Northern Trust Quality Low Volatility Index℠

rr_AverageAnnualReturnSinceInception

22.52%

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rr_AverageAnnualReturnYear01

26.38%

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