v3.26.1
Shareholder Report
 6 Months Ended
Jun. 30, 2026
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 Jun. 30, 2026
C000027306 [Member]  
Shareholder Report [Line Items]  
Fund Name AMG Veritas Asia Pacific Fund
Class Name Class I
Trading Symbol MSEIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about AMG Veritas Asia Pacific Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
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 Veritas Asia Pacific Fund
(Class I/MSEIX)		 $57 0.93%
Expenses Paid, Amount $ 57
Expense Ratio, Percent 0.93%
Net Assets $ 153,843,019
Holdings Count | Holding 32
Advisory Fees Paid, Amount $ 385,146
Investment Company Portfolio Turnover 108.00%
Additional Fund Statistics [Text Block]
Key Fund Statistics (as of June 30, 2026)
Fund net assets $153,843,019
Total number of portfolio holdings 32
Net advisory fees paid $385,146
Portfolio turnover rate as of the end of the reporting period 108%
Holdings [Text Block]
Graphical Representation of Holdings (as of June 30, 2026)
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
Samsung Electronics Co., Ltd. (South Korea) 13.2%
Taiwan Semiconductor Manufacturing Co., Ltd. (Taiwan) 11.3%
Delta Electronics, Inc. (Taiwan) 8.2%
MediaTek, Inc. (Taiwan) 4.4%
SK hynix, Inc. (South Korea) 4.1%
Accton Technology Corp. (Taiwan) 4.0%
Elite Material Co., Ltd. (Taiwan) 3.6%
Murata Manufacturing Co., Ltd. (Japan) 3.5%
HD Hyundai Heavy Industries Co., Ltd. (South Korea) 3.1%
Unimicron Technology Corp. (Taiwan) 2.5%
Top Ten as a Group 57.9%
Portfolio Breakdown
Graphical Representation - Allocation 1 Chart
Country Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Samsung Electronics Co., Ltd. (South Korea) 13.2%
Taiwan Semiconductor Manufacturing Co., Ltd. (Taiwan) 11.3%
Delta Electronics, Inc. (Taiwan) 8.2%
MediaTek, Inc. (Taiwan) 4.4%
SK hynix, Inc. (South Korea) 4.1%
Accton Technology Corp. (Taiwan) 4.0%
Elite Material Co., Ltd. (Taiwan) 3.6%
Murata Manufacturing Co., Ltd. (Japan) 3.5%
HD Hyundai Heavy Industries Co., Ltd. (South Korea) 3.1%
Unimicron Technology Corp. (Taiwan) 2.5%
Top Ten as a Group 57.9%
C000027305 [Member]  
Shareholder Report [Line Items]  
Fund Name AMG Veritas Asia Pacific Fund
Class Name Class N
Trading Symbol MGSEX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about AMG Veritas Asia Pacific Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
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 Veritas Asia Pacific Fund
(Class N/MGSEX)		 $72 1.17%
Expenses Paid, Amount $ 72
Expense Ratio, Percent 1.17%
Net Assets $ 153,843,019
Holdings Count | Holding 32
Advisory Fees Paid, Amount $ 385,146
Investment Company Portfolio Turnover 108.00%
Additional Fund Statistics [Text Block]
Key Fund Statistics (as of June 30, 2026)
Fund net assets $153,843,019
Total number of portfolio holdings 32
Net advisory fees paid $385,146
Portfolio turnover rate as of the end of the reporting period 108%
Holdings [Text Block]
Graphical Representation of Holdings (as of June 30, 2026)
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
Samsung Electronics Co., Ltd. (South Korea) 13.2%
Taiwan Semiconductor Manufacturing Co., Ltd. (Taiwan) 11.3%
Delta Electronics, Inc. (Taiwan) 8.2%
MediaTek, Inc. (Taiwan) 4.4%
SK hynix, Inc. (South Korea) 4.1%
Accton Technology Corp. (Taiwan) 4.0%
Elite Material Co., Ltd. (Taiwan) 3.6%
Murata Manufacturing Co., Ltd. (Japan) 3.5%
HD Hyundai Heavy Industries Co., Ltd. (South Korea) 3.1%
Unimicron Technology Corp. (Taiwan) 2.5%
Top Ten as a Group 57.9%
Portfolio Breakdown
Graphical Representation - Allocation 1 Chart
Country Allocation
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
Samsung Electronics Co., Ltd. (South Korea) 13.2%
Taiwan Semiconductor Manufacturing Co., Ltd. (Taiwan) 11.3%
Delta Electronics, Inc. (Taiwan) 8.2%
MediaTek, Inc. (Taiwan) 4.4%
SK hynix, Inc. (South Korea) 4.1%
Accton Technology Corp. (Taiwan) 4.0%
Elite Material Co., Ltd. (Taiwan) 3.6%
Murata Manufacturing Co., Ltd. (Japan) 3.5%
HD Hyundai Heavy Industries Co., Ltd. (South Korea) 3.1%
Unimicron Technology Corp. (Taiwan) 2.5%
Top Ten as a Group 57.9%
C000027310 [Member]  
Shareholder Report [Line Items]  
Fund Name AMG GW&K ESG Bond Fund
Class Name Class N
Trading Symbol MGFIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about AMG GW&K ESG Bond Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
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 ESG Bond Fund
(Class N/MGFIX)		 $34 0.68%
Expenses Paid, Amount $ 34
Expense Ratio, Percent 0.68%
Net Assets $ 313,740,586
Holdings Count | Holding 183
Advisory Fees Paid, Amount $ 299,878
Investment Company Portfolio Turnover 8.00%
Additional Fund Statistics [Text Block]
Key Fund Statistics (as of June 30, 2026)
Fund net assets $313,740,586
Total number of portfolio holdings 183
Net advisory fees paid $299,878
Portfolio turnover rate as of the end of the reporting period 8%
Holdings [Text Block]
Graphical Representation of Holdings (as of June 30, 2026)
Top ten holdings and portfolio breakdown are shown as a percentage of net assets of the Fund and ratings are shown as a percentage of total long-term investments of the Fund.
Top Ten Holdings
U.S. Treasury Bonds, 2.250%, 05/15/41 3.6%
U.S. Treasury Bonds, 3.125%, 05/15/48 2.6%
FNMA, 3.500%, 08/01/49 2.2%
FNMA, 3.500%, 02/01/47 1.9%
FNMA, 3.500%, 02/01/35 1.8%
FNMA, 4.000%, 06/01/48 1.8%
U.S. Treasury Bonds, 3.625%, 02/15/53 1.7%
FNMA, 5.500%, 11/01/52 1.5%
FNMA, 4.000%, 07/01/44 1.3%
FHLMC, 3.000%, 11/01/49 1.1%
Top Ten as a Group 19.5%
Portfolio Breakdown
Graphical Representation - Allocation 1 Chart
Ratings
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
U.S. Treasury Bonds, 2.250%, 05/15/41 3.6%
U.S. Treasury Bonds, 3.125%, 05/15/48 2.6%
FNMA, 3.500%, 08/01/49 2.2%
FNMA, 3.500%, 02/01/47 1.9%
FNMA, 3.500%, 02/01/35 1.8%
FNMA, 4.000%, 06/01/48 1.8%
U.S. Treasury Bonds, 3.625%, 02/15/53 1.7%
FNMA, 5.500%, 11/01/52 1.5%
FNMA, 4.000%, 07/01/44 1.3%
FHLMC, 3.000%, 11/01/49 1.1%
Top Ten as a Group 19.5%
C000125498 [Member]  
Shareholder Report [Line Items]  
Fund Name AMG GW&K ESG Bond Fund
Class Name Class I
Trading Symbol MGBIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about AMG GW&K ESG Bond Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
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 ESG Bond Fund
(Class I/MGBIX)		 $24 0.48%
Expenses Paid, Amount $ 24
Expense Ratio, Percent 0.48%
Net Assets $ 313,740,586
Holdings Count | Holding 183
Advisory Fees Paid, Amount $ 299,878
Investment Company Portfolio Turnover 8.00%
Additional Fund Statistics [Text Block]
Key Fund Statistics (as of June 30, 2026)
Fund net assets $313,740,586
Total number of portfolio holdings 183
Net advisory fees paid $299,878
Portfolio turnover rate as of the end of the reporting period 8%
Holdings [Text Block]
Graphical Representation of Holdings (as of June 30, 2026)
Top ten holdings and portfolio breakdown are shown as a percentage of net assets of the Fund and ratings are shown as a percentage of total long-term investments of the Fund.
Top Ten Holdings
U.S. Treasury Bonds, 2.250%, 05/15/41 3.6%
U.S. Treasury Bonds, 3.125%, 05/15/48 2.6%
FNMA, 3.500%, 08/01/49 2.2%
FNMA, 3.500%, 02/01/47 1.9%
FNMA, 3.500%, 02/01/35 1.8%
FNMA, 4.000%, 06/01/48 1.8%
U.S. Treasury Bonds, 3.625%, 02/15/53 1.7%
FNMA, 5.500%, 11/01/52 1.5%
FNMA, 4.000%, 07/01/44 1.3%
FHLMC, 3.000%, 11/01/49 1.1%
Top Ten as a Group 19.5%
Portfolio Breakdown
Graphical Representation - Allocation 1 Chart
Ratings
Graphical Representation - Allocation 2 Chart
Largest Holdings [Text Block]
Top Ten Holdings
U.S. Treasury Bonds, 2.250%, 05/15/41 3.6%
U.S. Treasury Bonds, 3.125%, 05/15/48 2.6%
FNMA, 3.500%, 08/01/49 2.2%
FNMA, 3.500%, 02/01/47 1.9%
FNMA, 3.500%, 02/01/35 1.8%
FNMA, 4.000%, 06/01/48 1.8%
U.S. Treasury Bonds, 3.625%, 02/15/53 1.7%
FNMA, 5.500%, 11/01/52 1.5%
FNMA, 4.000%, 07/01/44 1.3%
FHLMC, 3.000%, 11/01/49 1.1%
Top Ten as a Group 19.5%