S000049941 [Member] Investment Strategy - AMG GW&K SMALL/MID CAP CORE FUND	Dec. 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. The Fund primarily invests in common stock and preferred stock of U.S. small- and mid-capitalization companies. Small- and mid-capitalization companies are those with a market capitalization, at the time of purchase, between $250 million and $10 billion or otherwise within the range of capitalizations of companies in the Russell 2500™ 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 threshold. Because of this, the Fund may have less than 80% of its net assets in equity securities of small- and mid-capitalization companies at any given time. The Fund invests in an unrestricted opportunity set, pursuing what it believes to be quality companies with either growth- or value-oriented characteristics. GW&K Investment Management, LLC, the subadviser to the Fund (“GW&K” or the “Subadviser”), seeks to assemble a portfolio of securities diversified as to companies and industries. The Subadviser may consider increasing or reducing the Fund’s investment in a particular industry in view of the Fund’s goal of achieving industry diversification.

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. The Fund primarily invests in common stock and preferred stock of U.S. small- and mid-capitalization companies. Small- and mid-capitalization companies are those with a market capitalization, at the time of purchase, between $250 million and $10 billion or otherwise within the range of capitalizations of companies in the Russell 2500™ 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 threshold. Because of this, the Fund may have less than 80% of its net assets in equity securities of small- and mid-capitalization companies at any given time.

The Fund invests in an unrestricted opportunity set, pursuing what it believes to be quality companies with either growth- or value-oriented characteristics. GW&K Investment Management, LLC, the subadviser to the Fund (“GW&K” or the “Subadviser”), seeks to assemble a portfolio of securities diversified as to companies and industries. The Subadviser may consider increasing or reducing the Fund’s investment in a particular industry in view of the Fund’s goal of achieving industry diversification.