S000016733 [Member] Investment Strategy - AMG River Road Small-Mid Cap Value Fund	Oct. 31, 2025
Prospectus [Line Items]
Strategy [Heading]	Principal Investment Strategies
Strategy Narrative [Text Block]	Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in equity securities of small- and mid-capitalization companies that River Road Asset Management, LLC, the subadviser to the Fund (“River Road” or the “Subadviser”), believes are undervalued. The Fund currently considers small- and mid-capitalization companies to be those with market capitalizations at the time of acquisition within the range of capitalizations of companies in the Russell 2500TM Index (between $119.4 million and $18.6 billion as of April 30, 2025, the date of the latest reconstitution of the Index (implemented by the Index June 27, 2025)). This capitalization range will change over time. The Fund may continue to hold securities of a portfolio company that subsequently drops below or appreciates above this capitalization range. Because of this, the Fund may have less than 80% of its net assets in securities of small- and mid-cap companies at any given time. Value investing involves buying stocks that the Subadviser believes are out of favor and/or undervalued in comparison to their peers or their prospects for growth. The Fund may invest in real estate investment trusts (“REITs”), convertible preferred stocks, and foreign securities (directly and through depositary receipts). The Fund may also invest in common stock of companies with market capitalizations that exceed that of the Russell 2500TM Index at the time of acquisition. The Subadviser’s investment philosophy is based upon its proprietary Absolute Value® approach, which seeks to generate attractive, sustainable returns over the long term, with an emphasis on minimizing downside portfolio risk. The Subadviser builds the Fund’s portfolio from the bottom up, making security-specific research central to the Subadviser’s process. At the core of the Subadviser’s Absolute Value® approach is a systematic method for assessing the ‘risk-to-reward’ characteristics of an investment. The goal of the research process is to formulate two outputs from which an investment decision is made – conviction rating (risk) and discount to value (reward). The Subadviser combines a stock’s conviction rating with its discount to value in an effort to determine not only whether the stock qualifies for investment, but also how the stock will be sized within the Fund. The Subadviser employs a balanced approach to diversification and a structured sell discipline that seeks to reduce portfolio volatility and the risk of permanent loss of capital.

Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in equity securities of small- and mid-capitalization companies that River Road Asset Management, LLC, the subadviser to the Fund (“River Road” or the “Subadviser”),

believes are undervalued. The Fund currently considers small- and mid-capitalization companies to be those with market capitalizations at the time of acquisition within the range of capitalizations of companies in the Russell 2500TM Index (between $119.4 million and $18.6 billion as of April 30, 2025, the date of the latest reconstitution of the Index (implemented by the Index June 27, 2025)). This capitalization range will change over time. The Fund may continue to hold securities of a portfolio company that subsequently drops below or appreciates above this capitalization range. Because of this, the Fund may have less than 80% of its net assets in securities of small- and mid-cap companies at any given time. Value investing involves buying stocks that the Subadviser believes are out of favor and/or undervalued in comparison to their peers or their prospects for growth. The Fund may invest in real estate investment trusts (“REITs”), convertible preferred stocks, and foreign securities (directly and through depositary receipts). The Fund may also invest in common stock of companies with market capitalizations that exceed that of the Russell 2500TM Index at the time of acquisition.

The Subadviser builds the Fund’s portfolio from the bottom up, making security-specific research central to the Subadviser’s process. At the core of the Subadviser’s Absolute Value® approach is a systematic method for assessing the ‘risk-to-reward’ characteristics of an investment. The goal of the research process is to formulate two outputs from which an investment decision is made – conviction rating (risk) and discount to value (reward). The Subadviser combines a stock’s conviction rating with its discount to value in an effort to determine not only whether the stock qualifies for investment, but also how the stock will be sized within the Fund.