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Shareholder Report
 6 Months Ended
Nov. 30, 2025
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name AMG Funds III
Entity Central Index Key 0000720309
Entity Investment Company Type N-1A
Document Period End Date Nov. 30, 2025
C000188985 [Member]  
Shareholder Report [Line Items]  
Fund Name AMG GW&K International Small Cap Fund
Class Name Class Z
Trading Symbol MECZX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about AMG GW&K International Small Cap Fund (the “Fund”) for the period of June 1, 2025 to November 30, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at https://wealth.amg.com/literature. You can also request this information by contacting us at 800.548.4539.
Additional Information Phone Number 800.548.4539
Additional Information Website https://wealth.amg.com/literature
Expenses [Text Block]
Fund Expenses
What were the Fund costs for the last six months?
(Based on a hypothetical $10,000 investment)
Fund (Class) Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
AMG GW&K International Small Cap Fund
(Class Z/MECZX)		 $46 0.89%
Expenses Paid, Amount $ 46
Expense Ratio, Percent 0.89%
Net Assets $ 43,503,617
Holdings Count | Holding 76
Advisory Fees Paid, Amount $ 62,730
Investment Company Portfolio Turnover 15.00%
Additional Fund Statistics [Text Block]
Key Fund Statistics (as of November 30, 2025)
Fund net assets $43,503,617
Total number of portfolio holdings 76
Net advisory fees paid $62,730
Portfolio turnover rate as of the end of the reporting period 15%
Holdings [Text Block]
Graphical Representation of Holdings (as of November 30, 2025)
Top ten holdings and portfolio breakdown are shown as a percentage of net assets of the Fund and country allocation is shown as a percentage of total long-term investments of the Fund.
Top Ten Holdings
Tel Aviv Stock Exchange, Ltd. (Israel) 2.4%
Gift Holdings, Inc. (Japan) 2.3%
Phoenix Financial, Ltd. (Israel) 2.2%
Senshu Electric Co., Ltd. (Japan) 2.2%
Elopak A.S.A. (Norway) 2.0%
Digital Information Technologies Corp. (Japan) 2.0%
Kitron A.S.A. (Norway) 1.9%
Garrett Motion, Inc. (Switzerland) 1.9%
Niterra Co., Ltd. (Japan) 1.9%
Sarantis, S.A. (Greece) 1.9%
Top Ten as a Group 20.7%
Portfolio Breakdown
Graphical Representation - Allocation 1 Chart
Country Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Tel Aviv Stock Exchange, Ltd. (Israel) 2.4%
Gift Holdings, Inc. (Japan) 2.3%
Phoenix Financial, Ltd. (Israel) 2.2%
Senshu Electric Co., Ltd. (Japan) 2.2%
Elopak A.S.A. (Norway) 2.0%
Digital Information Technologies Corp. (Japan) 2.0%
Kitron A.S.A. (Norway) 1.9%
Garrett Motion, Inc. (Switzerland) 1.9%
Niterra Co., Ltd. (Japan) 1.9%
Sarantis, S.A. (Greece) 1.9%
Top Ten as a Group 20.7%
C000091188 [Member]  
Shareholder Report [Line Items]  
Fund Name AMG GW&K International Small Cap Fund
Class Name Class I
Trading Symbol MECIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about AMG GW&K International Small Cap Fund (the “Fund”) for the period of June 1, 2025 to November 30, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at https://wealth.amg.com/literature. You can also request this information by contacting us at 800.548.4539.
Additional Information Phone Number 800.548.4539
Additional Information Website https://wealth.amg.com/literature
Expenses [Text Block]
Fund Expenses
What were the Fund costs for the last six months?
(Based on a hypothetical $10,000 investment)
Fund (Class) Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
AMG GW&K International Small Cap Fund
(Class I/MECIX)		 $51 0.99%
Expenses Paid, Amount $ 51
Expense Ratio, Percent 0.99%
Net Assets $ 43,503,617
Holdings Count | Holding 76
Advisory Fees Paid, Amount $ 62,730
Investment Company Portfolio Turnover 15.00%
Additional Fund Statistics [Text Block]
Key Fund Statistics (as of November 30, 2025)
Fund net assets $43,503,617
Total number of portfolio holdings 76
Net advisory fees paid $62,730
Portfolio turnover rate as of the end of the reporting period 15%
Holdings [Text Block]
Graphical Representation of Holdings (as of November 30, 2025)
Top ten holdings and portfolio breakdown are shown as a percentage of net assets of the Fund and country allocation is shown as a percentage of total long-term investments of the Fund.
Top Ten Holdings
Tel Aviv Stock Exchange, Ltd. (Israel) 2.4%
Gift Holdings, Inc. (Japan) 2.3%
Phoenix Financial, Ltd. (Israel) 2.2%
Senshu Electric Co., Ltd. (Japan) 2.2%
Elopak A.S.A. (Norway) 2.0%
Digital Information Technologies Corp. (Japan) 2.0%
Kitron A.S.A. (Norway) 1.9%
Garrett Motion, Inc. (Switzerland) 1.9%
Niterra Co., Ltd. (Japan) 1.9%
Sarantis, S.A. (Greece) 1.9%
Top Ten as a Group 20.7%
Portfolio Breakdown
Graphical Representation - Allocation 1 Chart
Country Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Tel Aviv Stock Exchange, Ltd. (Israel) 2.4%
Gift Holdings, Inc. (Japan) 2.3%
Phoenix Financial, Ltd. (Israel) 2.2%
Senshu Electric Co., Ltd. (Japan) 2.2%
Elopak A.S.A. (Norway) 2.0%
Digital Information Technologies Corp. (Japan) 2.0%
Kitron A.S.A. (Norway) 1.9%
Garrett Motion, Inc. (Switzerland) 1.9%
Niterra Co., Ltd. (Japan) 1.9%
Sarantis, S.A. (Greece) 1.9%
Top Ten as a Group 20.7%
C000091187 [Member]  
Shareholder Report [Line Items]  
Fund Name AMG GW&K International Small Cap Fund
Class Name Class N
Trading Symbol MECAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about AMG GW&K International Small Cap Fund (the “Fund”) for the period of June 1, 2025 to November 30, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at https://wealth.amg.com/literature. You can also request this information by contacting us at 800.548.4539.
Additional Information Phone Number 800.548.4539
Additional Information Website https://wealth.amg.com/literature
Expenses [Text Block]
Fund Expenses
What were the Fund costs for the last six months?
(Based on a hypothetical $10,000 investment)
Fund (Class) Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment
AMG GW&K International Small Cap Fund
(Class N/MECAX)		 $59 1.14%
Expenses Paid, Amount $ 59
Expense Ratio, Percent 1.14%
Net Assets $ 43,503,617
Holdings Count | Holding 76
Advisory Fees Paid, Amount $ 62,730
Investment Company Portfolio Turnover 15.00%
Additional Fund Statistics [Text Block]
Key Fund Statistics (as of November 30, 2025)
Fund net assets $43,503,617
Total number of portfolio holdings 76
Net advisory fees paid $62,730
Portfolio turnover rate as of the end of the reporting period 15%
Holdings [Text Block]
Graphical Representation of Holdings (as of November 30, 2025)
Top ten holdings and portfolio breakdown are shown as a percentage of net assets of the Fund and country allocation is shown as a percentage of total long-term investments of the Fund.
Top Ten Holdings
Tel Aviv Stock Exchange, Ltd. (Israel) 2.4%
Gift Holdings, Inc. (Japan) 2.3%
Phoenix Financial, Ltd. (Israel) 2.2%
Senshu Electric Co., Ltd. (Japan) 2.2%
Elopak A.S.A. (Norway) 2.0%
Digital Information Technologies Corp. (Japan) 2.0%
Kitron A.S.A. (Norway) 1.9%
Garrett Motion, Inc. (Switzerland) 1.9%
Niterra Co., Ltd. (Japan) 1.9%
Sarantis, S.A. (Greece) 1.9%
Top Ten as a Group 20.7%
Portfolio Breakdown
Graphical Representation - Allocation 1 Chart
Country Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Tel Aviv Stock Exchange, Ltd. (Israel) 2.4%
Gift Holdings, Inc. (Japan) 2.3%
Phoenix Financial, Ltd. (Israel) 2.2%
Senshu Electric Co., Ltd. (Japan) 2.2%
Elopak A.S.A. (Norway) 2.0%
Digital Information Technologies Corp. (Japan) 2.0%
Kitron A.S.A. (Norway) 1.9%
Garrett Motion, Inc. (Switzerland) 1.9%
Niterra Co., Ltd. (Japan) 1.9%
Sarantis, S.A. (Greece) 1.9%
Top Ten as a Group 20.7%