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Investment Strategy - Natixis Funds Trust I
Dec. 31, 2025
Mirova Global Megatrends Fund  
Prospectus [Line Items]  
Strategy [Heading] Investments, Risks and PerformancePrincipal Investment Strategies
Strategy Narrative [Text Block]
Under normal circumstances, the Fund will invest at least 80% of its net assets (plus any borrowings made for investment purposes) in equity securities. Equity securities may include common stocks, preferred stocks, depositary receipts and real estate investment trusts (“REITs”). The Fund invests in securities of companies located in no fewer than three countries, which may include the U.S. Under normal circumstances, the Fund will invest a percentage of its assets in securities of companies located outside the U.S. equal to at least the lesser of 40% or the percentage of foreign issuers in the Fund’s benchmark, the MSCI World Index, less 5%. The percentage of the Fund’s investments in foreign securities is at least partially based on the composition of the Fund’s benchmark. As a result, the Fund’s exposure to securities of companies located outside the U.S. may fluctuate in connection with variations in the foreign exposure of the Fund’s benchmark. The Fund may invest up to 25% of its assets in securities of companies located in emerging markets. Emerging markets are economies that the Adviser believes are not generally recognized to be fully developed markets, as measured by gross national income, financial market infrastructure, market capitalization and/or other factors. The Fund may invest in growth and value companies of any size and may also invest in initial public offerings.
The Adviser focuses on megatrends, which are trends that are global and long-term in nature and that will lead to significant transitions in the global economy. The Adviser organizes these global megatrends into four major transitions: (i) demographics; (ii) technology; (iii) environment; and (iv) governance. The Adviser reviews the firm’s convictions around the four major transitions annually. Given the breadth and long-term nature of the transitions, they are unlikely to change. Investable megatrends within the transitions, however, are frequently evaluated.
In making its investment decisions, the Adviser uses a bottom-up approach focused on individual companies, rather than making pre-determined or top-down allocations to specific themes, industries or economic factors. The Adviser may sell a security due to a deterioration in the company’s fundamental quality, a change in megatrend exposure or sustainability opinion, a controversy alert such as one relating to human rights, or if the Adviser believes the security has little potential for price appreciation or there is greater relative value in other securities in the Fund’s investment universe.
Vaughan Nelson Small Cap Fund  
Prospectus [Line Items]  
Strategy [Heading] Investments, Risks and PerformancePrincipal Investment Strategies
Strategy Narrative [Text Block]
Under normal circumstances, the Fund will invest at least 80% of its net assets (plus any borrowings made for investment purposes) in the equity securities, including common stocks and preferred stocks, of “small-capitalization companies.” Equity securities may take the form of stock in corporations, limited partnership interests, interests in limited liability companies, real estate investment trusts (“REITs”) or other trusts and other similar securities representing direct or indirect ownership interests in business organizations. Currently, the Fund defines a small-capitalization company to be one whose market capitalization, at the time of purchase, either falls within the capitalization range of the Russell 2000® Value Index or is $3.5 billion or less. While the market capitalization range for the Russell 2000® Value Index fluctuates, at December 31, 2025, it was $5.61 million to $17.0 billion. The Fund may, however, invest in companies with large-capitalizations. 
Vaughan Nelson Investment Management, L.P. (“Vaughan Nelson”) invests in small-capitalization companies with a focus on those companies meeting Vaughan Nelson’s return expectations. Vaughan Nelson uses a bottom-up value oriented investment process in constructing the Fund’s portfolio. Vaughan Nelson seeks companies with the following characteristics, although not all of the companies selected will have these attributes:
Companies earning a positive return on capital with stable-to-improving returns.
Companies valued at a discount to their asset value.
Companies with an attractive and sustainable dividend level.
In selecting investments for the Fund, Vaughan Nelson generally employs the following strategies:
Value-driven investment philosophy that selects stocks selling at a relatively low value based on discounted cash flow models. Vaughan Nelson selects companies that it believes are out of favor or misunderstood.
Vaughan Nelson starts with an investment universe of 5,000 securities. Vaughan Nelson then uses value-driven screens to create a research universe of companies with market capitalizations of at least $100 million.
Vaughan Nelson uses fundamental analysis to construct a portfolio of 60 to 80 securities that, in the opinion of Vaughan Nelson, is made up of quality companies with the potential to provide significant increases in share price over a three year period.
Vaughan Nelson will generally sell a security when it reaches Vaughan Nelson’s price target or when the issuer shows a change in financial condition, competitive pressures, poor management decisions or internal or external forces reducing future expected returns from those expected at the time of investment.
The Fund may also:
Invest in convertible preferred stock and convertible debt securities.
Invest in foreign securities, including emerging market securities.
Invest in REITs.
Invest in securities offered in initial public offerings (“IPOs”).
Natixis Oakmark International Fund  
Prospectus [Line Items]  
Strategy [Heading] Investments, Risks and PerformancePrincipal Investment Strategies
Strategy Narrative [Text Block]
The Fund invests primarily in a diversified portfolio of common stocks of non-U.S. companies. The Fund may invest in non-U.S. markets throughout the world, including emerging markets. Ordinarily, the Fund will invest in the securities of at least five countries outside the U.S. There are no geographic limits on the Fund’s non-U.S. investments. Although the Fund invests primarily in common stocks of non-U.S. companies it may also invest in the securities of U.S. companies. The Fund may invest in the securities of small-, mid- and large-capitalization companies.
The Fund’s subadviser, Harris Associates L.P. (“Harris Associates”) uses a value investment philosophy in selecting equity securities, such as common stocks, preferred stocks, warrants, and securities convertible into common stocks and preferred stocks. This value investment philosophy is based upon the belief that, over time, a company’s stock price converges with Harris Associates’ estimate of its intrinsic value. By “intrinsic value,” Harris Associates means its estimate of the price a knowledgeable buyer would pay to acquire the entire business. Harris Associates believes that investing in securities priced significantly below what Harris Associates believes is a company’s intrinsic value presents the best opportunity to achieve the Fund’s investment objective.
Harris Associates uses this value investment philosophy to identify companies that have discounted stock prices compared to what Harris Associates believes are the companies’ intrinsic values. In assessing such companies, Harris Associates looks for the following characteristics, although not all of the companies selected will have these attributes: (1) free cash flows and intelligent investment of excess cash; (2) earnings that are growing and are reasonably predictable; and (3) high level of company management alignment with shareholders.
Once Harris Associates identifies a stock that it believes is selling at a significant discount to Harris Associates’ estimated intrinsic value and that the issuer has one or more of the additional qualities mentioned above, Harris Associates generally will consider buying that security for the Fund. Harris Associates usually sells a security when the price approaches its estimated value or the issuer’s fundamentals change. Harris Associates monitors each holding and adjusts its price targets as warranted to reflect changes in the issuer’s fundamentals. The Fund’s portfolio typically holds 30 to 65 stocks.
Natixis U.S. Equity Opportunities Fund  
Prospectus [Line Items]  
Strategy [Heading] Investments, Risks and PerformancePrincipal Investment Strategies
Strategy Narrative [Text Block]
Under normal circumstances, the Fund will invest at least 80% of its net assets (plus any borrowings made for investment purposes) in equity securities. Equity securities may include  common stocks and preferred stocks. Under normal circumstances, the Fund will invest at least 80% of its net assets (plus any borrowings made for investment purposes) in securities of U.S. issuers. The Fund’s approach to equity investing combines the styles of two subadvisers in selecting securities for each of the Fund’s sleeves. The sleeves and their subadvisers are listed below.
Harris Associates - Large Cap Value sleeve - Under normal circumstances, the Large Cap Value sleeve of the Fund managed by Harris Associates L.P. (“Harris Associates”) will invest primarily in the common stocks of larger-capitalization companies that Harris Associates believes are trading at a substantial discount to the company’s “intrinsic value.” By “intrinsic value,” Harris Associates means its estimate of the price a knowledgeable buyer would pay to acquire the entire business. Harris Associates believes that investing in securities priced significantly below what Harris Associates believes is a company’s intrinsic value presents the best opportunity to achieve the Fund’s investment objectives. Harris Associates usually sells a security when the price approaches its estimated value and monitors each holding and adjusts its price targets as warranted to reflect changes in the issuer’s fundamentals.  In determining whether an issuer is a U.S. or foreign issuer for the Harris Associates – Large Cap Value sleeve, Harris Associates considers various factors, including its country of domicile, the primary stock exchange on which it trades, the location from which the majority of its revenue comes, and its reporting currency.
Loomis Sayles - All Cap Growth sleeve - Under normal circumstances, the All Cap Growth sleeve of the Fund, managed by Loomis, Sayles & Company, L.P. (“Loomis Sayles”), will invest primarily in equity securities, including common stocks and depositary receipts. This sleeve may invest in companies of any size. The sleeve normally invests across a wide range of sectors and industries. The sleeve’s portfolio manager employs a growth style of equity management that emphasizes companies with sustainable competitive advantages versus others, long-term structural growth drivers that will lead to above-average future cash flow growth, attractive cash flow returns on invested capital, and management teams focused on creating long-term value for shareholders. The sleeve’s portfolio manager aims to invest in companies when they trade at a significant discount to the estimate of intrinsic value (i.e. companies with share prices trading significantly below what the portfolio manager believes the share price should be). The sleeve will consider selling a portfolio investment when the portfolio manager believes an unfavorable structural change occurs within a given business or the markets in which it operates, a critical underlying investment assumption is flawed, when a more attractive reward-to-risk opportunity becomes available, when the portfolio manager believes the current price fully reflects intrinsic value, or for other investment reasons which the portfolio manager deems appropriate.  Although certain equity securities purchased by the Loomis Sayles – All Cap Growth sleeve of the Fund may be issued by domestic companies incorporated outside of the United States, Loomis Sayles uses a propriety definition to determine whether a security is classified as U.S. or non-U.S. In determining the location of an issuer for these purposes, or where the issuer’s principal activities are based, Loomis Sayles will consider a variety of factors (collectively designed to assess whether an issuer is economically tied to a particular country or region), including but not limited to: the markets in which the issuer’s securities are primarily traded; the location of the issuer’s headquarters, principal offices, or operations; the country where the issuer is organized; the percentage of the issuer’s revenues or profits derived from goods produced or sold, investments made, or services performed in the relevant country; and information provided by third-party data analytics service providers. No single factor will necessarily be determinative, nor must all factors be present for Loomis Sayles to assess an issuer’s location. Loomis Sayles may assign different weights to these factors based on different geographic policies, countries, or products.
Subject to the allocation policy adopted by the Fund’s Board of Trustees, Natixis Advisors generally allocates capital invested in the Fund equally (i.e., 50%) between its two sleeves. Under the allocation policy, Natixis Advisors may also allocate capital away from or towards each sleeve from time to time and may reallocate capital between the sleeves. Each subadviser manages its sleeve of the Fund’s assets in accordance with its distinct investment style and strategy.
The Fund may also:
Invest in securities offered in initial public offerings (“IPOs”) and securities issued pursuant to Rule 144A under the Securities Act of 1933 (“Rule 144A securities”).
Invest in convertible preferred stock and convertible debt securities.
Invest in real estate investment trusts (“REITs”).
Invest in fixed-income securities, including U.S. government bonds and below-investment grade fixed-income securities (commonly known as “junk bonds”).
Hold securities of foreign issuers traded over-the-counter or on foreign exchanges, including securities in emerging markets and related currency hedging transactions.
Invest in equity securities of Canadian issuers.