Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	AMG FUNDS III
Prospectus Date	rr_ProspectusDate	May 01, 2020
AMG Managers Special Equity Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	AMG Managers Special Equity Fund
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The AMG Managers Special Equity Fund’s (the “Fund”) investment objective is to achieve long-term capital appreciation by investing in a diversified portfolio of equity securities of small- and medium-sized companies.
Expense [Heading]	rr_ExpenseHeading	FEES AND EXPENSES OF THE FUND
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. If you transact in Class I shares of the Fund through a financial intermediary, you may be required to pay a commission to the financial intermediary for effecting such transactions. Such commissions are charged by the financial intermediary and are not reflected in the table or Expense Example below.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	May 1, 2021
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	PORTFOLIO TURNOVER
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 96% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	96.00%
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Expense information has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	EXPENSE EXAMPLE
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example will help you compare the cost of investing in the Fund to the cost of investing in other mutual funds. The Example makes certain assumptions. It assumes that you invest $10,000 as an initial investment in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. It also assumes that your investment has a 5% total return each year and the Fund’s operating expenses remain the same. The Example includes the Fund’s contractual expense limitation through May 1, 2021. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:
Strategy [Heading]	rr_StrategyHeading	PRINCIPAL INVESTMENT STRATEGIES
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal conditions, the Fund will invest at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in equity securities (generally, common and preferred stocks). Although the Fund focuses its investments in securities of small- and medium-capitalization companies pursuant to its investment objective, the Fund has historically invested substantially all of its assets in the securities of U.S. small-capitalization companies. Typically, the Fund invests in companies with capitalizations that are within the range of capitalizations of companies in the Russell 2000® Growth Index (the “Index”). As of May 10, 2019, the date of the latest reconstitution of the Index (implemented by the Index June 28, 2019), the range of market capitalizations for the Index was $44.1 million to $5.859 billion. This capitalization range will change over time. The Fund may purchase securities, or retain securities that it already has purchased, even if the securities are outside the Fund’s typical capitalization range. The Fund’s assets are currently allocated among four Subadvisers, each of which acts independently of the others and uses its own methodology to select portfolio investments. Specifically: The Subadvisers utilize a growth approach to investing whereby three of the four Subadvisers seek to identify companies that are exhibiting rapid growth in their businesses and the fourth Subadviser seeks to identify companies that possess better than average long-term growth rates combined with several other fundamental characteristics. Three of the four Subadvisers examine the underlying businesses, financial statements, competitive environment and company management in order to assess the future profitability of each company. One of the four Subadvisers utilizes a quantitative model driven by fundamental and technical stock selection variables, including relative value, profit trends, capital structure and price history. This process seeks to impose strict discipline over stock selection, unimpeded by market or manager psychology. A Subadviser may sell a security if it believes that the future profitability of a company does not support the current stock price. With the combination of the above strategies, the Fund seeks to generate returns from capital appreciation due to earnings growth along with improvements in the valuations of the securities such as, among other things, increases in the price to earnings ratios.
Risk [Heading]	rr_RiskHeading	PRINCIPAL RISKS
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	There is the risk that you may lose money on your investment. All investments carry a certain amount of risk, and the Fund cannot guarantee that it will achieve its investment objective. An investment in the Fund is not a deposit or obligation of any bank, is not endorsed or guaranteed by any bank, and is not insured by the Federal Deposit Insurance Corporation (“FDIC”) or any other government agency. Below are some of the risks of investing in the Fund. The risks are presented in an order intended to facilitate readability and their order does not imply that the realization of one risk is more likely to occur than another risk or likely to have a greater adverse impact than another risk. The significance of any specific risk to an investment in the Fund will vary over time, depending on the composition of the Fund’s portfolio, market conditions, and other factors. You should read all of the risk information presented below carefully, because any one or more of these risks may result in losses to the Fund. Market Risk—market prices of investments held by the Fund may fall rapidly or unpredictably due to a variety of factors, including economic, political, or market conditions, or other factors including terrorism, war, natural disasters and the spread of infectious illness or other public health issues, including epidemics or pandemics such as the COVID-19 outbreak in 2020, or in response to events that affect particular industries or companies. Small- and Mid-Capitalization Stock Risk—the stocks of small- and mid-capitalization companies often have greater price volatility, lower trading volume, and less liquidity than the stocks of larger, more established companies. Growth Stock Risk—the prices of equity securities of companies that are expected to experience relatively rapid earnings growth, or “growth stocks,” may be more sensitive to market movements because the prices tend to reflect future investor expectations rather than just current profits. Management Risk—because the Fund is an actively managed investment portfolio, security selection or focus on securities in a particular style, market sector or group of companies may cause the Fund to incur losses or underperform relative to its benchmarks or other funds with a similar investment objective. There can be no guarantee that a Subadviser’s investment techniques and risk analysis will produce the desired result. Liquidity Risk—the Fund may not be able to dispose of particular investments, such as illiquid securities, readily at favorable times or prices or the Fund may have to sell them at a loss. Foreign Investment Risk—investments in foreign issuers involve additional risks (such as risks arising from less frequent trading, changes in political or social conditions, and less publicly available information about non-U.S. issuers) that differ from those associated with investments in U.S. issuers and may result in greater price volatility. High Portfolio Turnover Risk—higher portfolio turnover may adversely affect Fund performance by increasing Fund transaction costs and may increase a shareholder’s tax liability. Model and Data Risk—when a quantitative model (“Model”) or information or data (“Data”) used in managing the Fund contains an error, or is incorrect or incomplete, any investment decision made in reliance on the Model or Data may not produce the desired results and the Fund may realize losses. In addition, any hedging based on a faulty Model or Data may prove to be unsuccessful. Furthermore, the success of a Model that is predictive in nature is dependent largely on the accuracy and reliability of the supplied historical data. All Models are susceptible to input errors or errors in design, which may cause the resulting output to be faulty. Sector Risk— issuers and companies that are in similar industry sectors may be similarly affected by particular economic or market events; to the extent the Fund has substantial holdings within a particular sector, the risks associated with that sector increase. Stocks in the information technology and health care sectors may comprise a significant portion of the Fund’s portfolio. The information technology sector may be affected by technological obsolescence, short product cycles, falling prices and profits, competitive pressures and general market conditions. Companies in the health care sector may be affected by government regulation, government approval of products and services, technological obsolescence, patent expirations, product liability or other litigation, and changes in governmental and private payment systems.
Risk Lose Money [Text]	rr_RiskLoseMoney	There is the risk that you may lose money on your investment.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the Fund is not a deposit or obligation of any bank, is not endorsed or guaranteed by any bank, and is not insured by the Federal Deposit Insurance Corporation (“FDIC”) or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	PERFORMANCE
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following performance information illustrates the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s performance compares to that of a broad based securities market index. As always, past performance of the Fund (before and after taxes) is not an indication of how the Fund will perform in the future. Effective February 27, 2017, outstanding Class S shares of the Fund (formerly Service Class shares, which were renamed Class S shares on October 1, 2016 (formerly Managers Class shares, which were renamed Service Class shares on April 1, 2013)) were renamed Class N shares. Effective October 1, 2016, outstanding Institutional Class shares of the Fund were renamed Class I shares. To obtain updated performance information please visit www.amgfunds.com or call 800.548.4539.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following performance information illustrates the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and by showing how the Fund’s performance compares to that of a broad based securities market index.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.548.4539
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	www.amgfunds.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As always, past performance of the Fund (before and after taxes) is not an indication of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Calendar Year Total Returns as of 12/31/19 (Class N)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter: 17.32% (4th Quarter 2010) Worst Quarter: -21.85% (4th Quarter 2018)
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns as of 12/31/19
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns are not relevant to investors who hold their Fund shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns are shown for Class N shares only, and after-tax returns for Class I shares will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns are not relevant to investors who hold their Fund shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”). After-tax returns are shown for Class N shares only, and after-tax returns for Class I shares will vary.
AMG Managers Special Equity Fund | Class N
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.90%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.51%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.41%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.05%)	[2]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.36%	[2]
1 Year	rr_ExpenseExampleYear01	$ 138
3 Years	rr_ExpenseExampleYear03	441
5 Years	rr_ExpenseExampleYear05	766
10 Years	rr_ExpenseExampleYear10	$ 1,687
2010	rr_AnnualReturn2010	33.11%
2011	rr_AnnualReturn2011	3.40%
2012	rr_AnnualReturn2012	10.35%
2013	rr_AnnualReturn2013	45.06%
2014	rr_AnnualReturn2014	1.22%
2015	rr_AnnualReturn2015	(0.54%)
2016	rr_AnnualReturn2016	13.11%
2017	rr_AnnualReturn2017	20.25%
2018	rr_AnnualReturn2018	(3.76%)
2019	rr_AnnualReturn2019	25.69%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2010
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	17.32%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2018
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(21.85%)
1 Year	rr_AverageAnnualReturnYear01	25.69%
5 Years	rr_AverageAnnualReturnYear05	10.35%
10 Years	rr_AverageAnnualReturnYear10	13.82%
AMG Managers Special Equity Fund | Class I
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.90%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.26%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.16%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.05%)	[2]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.11%	[2]
1 Year	rr_ExpenseExampleYear01	$ 113
3 Years	rr_ExpenseExampleYear03	364
5 Years	rr_ExpenseExampleYear05	633
10 Years	rr_ExpenseExampleYear10	$ 1,405
1 Year	rr_AverageAnnualReturnYear01	26.02%
5 Years	rr_AverageAnnualReturnYear05	10.63%
10 Years	rr_AverageAnnualReturnYear10	14.07%
AMG Managers Special Equity Fund | Return After Taxes on Distributions | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	18.13%
5 Years	rr_AverageAnnualReturnYear05	8.99%
10 Years	rr_AverageAnnualReturnYear10	13.12%
AMG Managers Special Equity Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	19.38%
5 Years	rr_AverageAnnualReturnYear05	8.08%
10 Years	rr_AverageAnnualReturnYear10	11.60%
AMG Managers Special Equity Fund | Russell 2000® Growth Index (reflects no deduction for fees, expenses, or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	28.48%
5 Years	rr_AverageAnnualReturnYear05	9.34%
10 Years	rr_AverageAnnualReturnYear10	13.01%

[1] Expense information has been restated to reflect current fees.

[2] AMG Funds LLC (the “Investment Manager”) has contractually agreed, through at least May 1, 2021, to waive management fees and/or pay or reimburse the Fund’s expenses in order to limit Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements (exclusive of taxes, interest (including interest incurred in connection with bank and custody overdrafts and in connection with securities sold short), shareholder servicing fees, distribution and service (12b-1) fees, brokerage commissions and other transaction costs, dividends payable with respect to securities sold short, acquired fund fees and expenses, and extraordinary expenses) of the Fund to the annual rate of 1.11% of the Fund’s average daily net assets (this annual rate or such other annual rate that may be in effect from time to time, the “Expense Cap”), subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months after the date any amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover such amounts from the Fund, provided that such repayment would not cause the Fund’s Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements (exclusive of the items noted in the parenthetical above) to exceed either (i) the Expense Cap in effect at the time such amounts were paid, waived or reimbursed, or (ii) the Expense Cap in effect at the time of such repayment by the Fund. The contractual expense limitation may only be terminated in the event the Investment Manager or a successor ceases to be the investment manager of the Fund or a successor fund, by mutual agreement between the Investment Manager and the AMG Funds III Board of Trustees or in the event of the Fund’s liquidation unless the Fund is reorganized or is a party to a merger in which the surviving entity is successor to the accounting and performance information of the Fund.