485BPOS

As filed with the Securities and Exchange Commission on May 14, 2018

1933 Act Registration No. 002-84012

1940 Act Registration No. 811-03752

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933	☒
Pre-Effective Amendment No.	☐
Post-Effective Amendment No. 119	☒

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940	☒
Amendment No. 120	☒

AMG FUNDS III

(Exact name of registrant as specified in charter)

600 Steamboat Road, Suite 300

Greenwich, Connecticut 06830

(Address of principal executive offices)

Registrant’s telephone number, including area code: (800) 835-3879

Gregory C. Davis

Ropes & Gray LLP

Three Embarcadero Center

San Francisco, CA 94111-4006

(Name and address of agent for service)

It is proposed that this filing will become effective:

☒ immediately upon filing pursuant to paragraph (b)

☐ on (date) pursuant to paragraph (b)

☐ 60 days after filing pursuant to paragraph (a)

☐ on (date) pursuant to paragraph (a)

☐ 75 days after filing pursuant to paragraph (a)(2)

☐ on (date) pursuant to (a)(2) of rule 485

If appropriate, check the following box:

☐ This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

The Amendment relates solely to AMG Managers Special Equity Fund, AMG Managers Global Income Opportunity Fund and AMG Managers Loomis Sayles Bond Fund, each a series of AMG Funds III.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, each as amended, AMG Funds III certifies that it meets all of the requirements for effectiveness of this registration statement under Rule 485(b) under the Securities Act of 1933, as amended, and has duly caused this amendment to the registration statement to be signed on its behalf by the undersigned, duly authorized, in the Town of Greenwich, and State of Connecticut, on the 14th day of May, 2018.

AMG FUNDS III
By:	/s/ Thomas Disbrow
	Thomas Disbrow
	Treasurer, Chief Financial Officer, and Principal Financial Officer

Pursuant to the requirements of the Securities Act, this amendment to the registration statement has been signed below by the following persons in the capacities and on the date indicated:

Signature	Title	Date
/s/ Bruce B. Bingham*	Trustee	May 14, 2018
Bruce B. Bingham
/s/ Christine C. Carsman*	Trustee	May 14, 2018
Christine C. Carsman
/s/ Edward J. Kaier*	Trustee	May 14, 2018
Edward J. Kaier
/s/ Kurt A. Keilhacker*	Trustee	May 14, 2018
Kurt A. Keilhacker
/s/ Steven J. Paggioli*	Trustee	May 14, 2018
Steven J. Paggioli
/s/ Richard F. Powers III*	Trustee	May 14, 2018
Richard F. Powers III
/s/ Eric Rakowski*	Trustee	May 14, 2018
Eric Rakowski
/s/ Victoria L. Sassine*	Trustee	May 14, 2018
Victoria L. Sassine
/s/ Thomas R. Schneeweis*	Trustee	May 14, 2018
Thomas R. Schneeweis
/s/ Keitha L. Kinne	President and	May 14, 2018
Keitha L. Kinne	Principal Executive Officer
	(Principal Executive Officer)
/s/ Thomas Disbrow	Treasurer,	May 14, 2018
Thomas Disbrow	Chief Financial Officer, and
	Principal Financial Officer
	(Principal Accounting Officer)
	(Principal Financial Officer)

*By:	/s/ Thomas Disbrow
	Thomas Disbrow

Pursuant to Powers of Attorney incorporated herein by reference to Post-Effective Amendment No. 118 to the Registrant’s Registration Statement on Form N-1A, Registration Nos. 002-84012 and 811-03752 (filed April 30, 2018).

Date: May 14, 2018

AMG FUNDS III

EXHIBIT INDEX

Exhibit No.	Description
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2017
Registrant Name	dei_EntityRegistrantName	AMG FUNDS III
Central Index Key	dei_EntityCentralIndexKey	0000720309
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Apr. 30, 2018
Document Effective Date	dei_DocumentEffectiveDate	May 01, 2018
Prospectus Date	rr_ProspectusDate	May 01, 2018

XML

All Share Classes | AMG Managers Special Equity Fund

AMG Managers Special Equity Fund

INVESTMENT OBJECTIVE

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.

FEES AND EXPENSES OF THE FUND

The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - All Share Classes - AMG Managers Special Equity Fund		Class N	Class I
Management Fee		0.90%	0.90%
Distribution and Service (12b-1) Fees		none	none
Other Expenses		0.51%	0.26%
Total Annual Fund Operating Expenses		1.41%	1.16%
Fee Waiver and Expense Reimbursements	[1]	(0.05%)	(0.05%)
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	[1]	1.36%	1.11%

EXPENSE EXAMPLE

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, 2019. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:

Expense Example - All Share Classes - AMG Managers Special Equity Fund - USD ($)	1 Year	3 Years	5 Years	10 Years
Class N	138	441	766	1,687
Class I	113	364	633	1,405

PORTFOLIO TURNOVER

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 81% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES

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 12, 2017, the date of the latest reconstitution of the Index (implemented by the Index June 23, 2017), the range of market capitalizations for the Index was $144 million to $4.36 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 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.

PRINCIPAL RISKS

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 described in alphabetical order and not in the order of importance or potential exposure.

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.

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.

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.

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.

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.

Market Risk—market prices of investments held by the Fund may fall rapidly or unpredictably due to a variety of factors, including changing economic, political, or market conditions or in response to events that affect particular industries or companies.

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.

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.

PERFORMANCE

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 800.835.3879.

Calendar Year Total Returns as of 12/31/17 (Class N)

Best Quarter: 19.39% (2nd Quarter 2009)
Worst Quarter: -27.92% (4th Quarter 2008)

Average Annual Total Returns as of 12/31/17

Average Annual Total Returns - All Share Classes - AMG Managers Special Equity Fund	1 Year	5 Years	10 Years
Class N	20.25%	14.71%	8.35%
Class N | Return After Taxes on Distributions	20.25%	14.71%	8.07%
Class N | Return After Taxes on Distributions and Sale of Fund Shares	11.46%	11.86%	6.70%
Class I	20.55%	14.91%	8.58%
Russell 2000® Growth Index (reflects no deduction for fees, expenses, or taxes)	22.17%	15.21%	9.19%

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. After-tax returns are shown for Class N shares only, and after-tax returns for Class I shares will vary.

[1] AMG Funds LLC (the “Investment Manager”) has contractually agreed, through at least May 1, 2019, to waive management fees and/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), shareholder servicing fees, brokerage commissions and other transaction costs, 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, subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months from the time such amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover from the Fund such fees waived and expenses paid, 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 the contractual expense limitation amount. 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.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	AMG FUNDS III
Prospectus Date	rr_ProspectusDate	May 01, 2018
All Share Classes | 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.
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, 2019
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 81% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	81.00%
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, 2019. 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 12, 2017, the date of the latest reconstitution of the Index (implemented by the Index June 23, 2017), the range of market capitalizations for the Index was $144 million to $4.36 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 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 described in alphabetical order and not in the order of importance or potential exposure. 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. 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. 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. 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. 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. Market Risk—market prices of investments held by the Fund may fall rapidly or unpredictably due to a variety of factors, including changing economic, political, or market conditions or in response to events that affect particular industries or companies. 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. 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.
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 800.835.3879.
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	800.835.3879
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 12/31/17 (Class N)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter: 19.39% (2nd Quarter 2009) Worst Quarter: -27.92% (4th Quarter 2008)
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns as of 12/31/17
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.
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. After-tax returns are shown for Class N shares only, and after-tax returns for Class I shares will vary.
All Share Classes | 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%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.41%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.05%)	[1]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.36%	[1]
1 Year	rr_ExpenseExampleYear01	$ 138
3 Years	rr_ExpenseExampleYear03	441
5 Years	rr_ExpenseExampleYear05	766
10 Years	rr_ExpenseExampleYear10	$ 1,687
2008	rr_AnnualReturn2008	(43.49%)
2009	rr_AnnualReturn2009	30.78%
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%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	19.39%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(27.92%)
1 Year	rr_AverageAnnualReturnYear01	20.25%
5 Years	rr_AverageAnnualReturnYear05	14.71%
10 Years	rr_AverageAnnualReturnYear10	8.35%
All Share Classes | 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%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.16%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.05%)	[1]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.11%	[1]
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	20.55%
5 Years	rr_AverageAnnualReturnYear05	14.91%
10 Years	rr_AverageAnnualReturnYear10	8.58%
All Share Classes | AMG Managers Special Equity Fund | Return After Taxes on Distributions | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	20.25%
5 Years	rr_AverageAnnualReturnYear05	14.71%
10 Years	rr_AverageAnnualReturnYear10	8.07%
All Share Classes | 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	11.46%
5 Years	rr_AverageAnnualReturnYear05	11.86%
10 Years	rr_AverageAnnualReturnYear10	6.70%
All Share Classes | 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	22.17%
5 Years	rr_AverageAnnualReturnYear05	15.21%
10 Years	rr_AverageAnnualReturnYear10	9.19%

[1] AMG Funds LLC (the “Investment Manager”) has contractually agreed, through at least May 1, 2019, to waive management fees and/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), shareholder servicing fees, brokerage commissions and other transaction costs, 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, subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months from the time such amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover from the Fund such fees waived and expenses paid, 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 the contractual expense limitation amount. 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.

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AMG Managers Loomis Sayles Bond Fund

AMG Managers Loomis Sayles Bond Fund

INVESTMENT OBJECTIVE

The AMG Managers Loomis Sayles Bond Fund’s (the “Fund”) investment objective is to achieve a high level of current income.

FEES AND EXPENSES OF THE FUND

The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund.

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses - AMG Managers Loomis Sayles Bond Fund		Class N	Class I
Management Fee		0.625%	0.625%
Distribution and Service (12b-1) Fees		none	none
Other Expenses	[1]	0.36%	0.26%
Total Annual Fund Operating Expenses		0.99%	0.89%

EXPENSE EXAMPLE

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. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:

Expense Example - AMG Managers Loomis Sayles Bond Fund - USD ($)	1 Year	3 Years	5 Years	10 Years
Class N	101	315	547	1,213
Class I	91	284	493	1,096

PORTFOLIO TURNOVER

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 4% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES

The Fund seeks to achieve its investment objective of a high level of current income from a diversified portfolio of fixed-income securities. Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in bonds (debt securities). Under normal market conditions, the Fund invests at least 65% of its total assets in investment grade corporate bonds, mortgage-related and other asset-backed securities and securities issued or guaranteed by the U.S. Government, its agencies or instrumentalities. Investment grade securities are rated at least in the BBB/Baa major rating category by Standard & Poor’s Ratings Services (“S&P”) or Moody’s Investors Service, Inc. (“Moody’s”) (or a similar rating from any nationally recognized statistical rating organization). The Fund may also invest in below investment grade bonds (commonly known as “junk bonds” or “high yield securities”) (those rated below BBB/Baa by S&P or Moody’s).

From time to time, the Fund may invest in bonds that are not rated by S&P, Moody’s or a similar nationally recognized statistical rating organization. In the event that the Fund invests in non-rated bonds, the equivalent rating determined by the Fund’s subadviser, Loomis, Sayles & Company, L.P. (the “Subadviser” or “Loomis”), will be used for the purpose of the Fund’s investment policies described above. Debt securities held by the Fund may have any remaining maturity. Occasionally, the Fund may purchase only the interest or principal component of a mortgage-related security. Up to 10% of the total assets of the Fund may be invested in non-U.S. dollar-denominated instruments.

Loomis applies fundamental investment research techniques when deciding which bonds to buy or sell. Typically, Loomis invests around three themes encompassing:

• Generally seeking fixed-income securities of issuers whose credit profiles it believes are improving.

• Significant use of securities whose price changes may not have a direct correlation with changes in interest rates. Loomis believes positive returns can be generated by having a portion of the Fund's assets invested in this type of security rather than primarily relying on interest rate changes to generate returns.

• Analysis of different sectors of the global economy and fixed income markets and differences in the yields ("spreads") of various fixed income securities in an effort to find securities that Loomis believes may produce attractive returns for the Fund in comparison to their risk with a preference for discounted securities with a yield advantage to the market average.

In deciding which securities to buy and sell, Loomis will consider, among other things, the financial strength of the issuer of the security, current spread/yield levels, Loomis’ expectations regarding general trends in interest rates, and comparisons of the level of risk associated with particular investments in conjunction with Loomis’ expectations concerning the potential return of those investments.

Loomis does not manage the Fund to maintain a given average duration, thus giving the Fund the flexibility to invest in securities with any remaining maturity as market conditions change. At times, the Fund’s average duration may be longer than that of the benchmark, so that the Fund is more sensitive to changes in interest rates than the benchmark. At other times the Fund’s average duration may be shorter than that of the benchmark, so that the Fund is less sensitive to changes in interest rates than the benchmark. Loomis may utilize futures contracts in order to adjust the Fund’s duration.

Loomis generally prefers securities that seek to protect against calls (early redemption by the issuer).

PRINCIPAL RISKS

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 described in alphabetical order and not in the order of importance or potential exposure.

Credit and Counterparty Risk—the issuer of bonds or other debt securities or a counterparty to a derivatives contract may be unable or unwilling, or may be perceived as unable or unwilling, to make timely interest, principal or settlement payments or otherwise honor its obligations.

Currency Risk—fluctuations in exchange rates may affect the total loss or gain on a non-U.S. dollar investment when converted back to U.S. dollars and exposure to non-U.S. currencies may subject the Fund to the risk that those currencies will decline in value relative to the U.S. dollar.

Derivatives Risk—the use of derivatives involves costs, the risk that the value of derivatives may not correlate perfectly with their underlying assets, rates or indices, and the risk of mispricing or improper valuation. The use of derivatives may not succeed for various reasons, and the complexity and rapidly changing structure of derivatives markets may increase the possibility of market losses.

Emerging Markets Risk—investments in emerging markets are subject to the general risks of foreign investments, as well as additional risks which can result in greater price volatility.

Extension Risk—during periods of rising interest rates, a debtor may pay back a bond or other fixed income security slower than expected or required, and the value of such security may fall.

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 Yield Risk—below investment grade debt securities and unrated securities of similar credit quality (commonly known as “junk bonds” or “high yield securities”) may be subject to greater levels of interest rate, credit, liquidity, and market risk than higher-rated securities. These securities are considered predominately speculative with respect to the issuer’s continuing ability to make principal and interest payments.

Inflation/Deflation Risk—Inflation risk is the risk that the value of assets or income from investments will be worth less in the future. Deflation risk is the risk that the prices throughout the economy decline over time—the opposite of inflation. Deflation may have an adverse effect on the creditworthiness of issuers and may make issuer default more likely, which may result in a decline in the value of the Fund’s portfolio.

Interest Rate Risk—fixed coupon payments (cash flows) of bonds and debt securities may become less competitive with the market in periods of rising interest rates and cause bond prices to decline. During periods of increasing interest rates, the Fund may experience high levels of volatility and shareholder redemptions, and may have to sell securities at times when it would otherwise not do so, and at unfavorable prices, which could reduce the returns of the Fund.

Leverage Risk—borrowing and some derivative investments such as futures and forward commitment transactions, may magnify smaller adverse market movements into relatively larger losses.

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.

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 the Subadviser’s investment techniques and risk analysis will produce the desired result.

Market Risk—market prices of investments held by the Fund may fall rapidly or unpredictably due to a variety of factors, including changing economic, political, or market conditions or in response to events that affect particular industries or companies.

Prepayment Risk—a debtor may exercise its right to pay back a bond or other debt security earlier than expected or required during periods of decreasing interest rates.

Reinvestment Risk—the Fund may have difficulty reinvesting payments from debtors and may receive lower rates than from its original investments.

PERFORMANCE

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 shares of the Fund’s sole share class, which were reclassified and redesignated as 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 800.835.3879.

Calendar Year Total Returns as of 12/31/17 (Class N)

Best Quarter: 16.27% (2nd Quarter 2009)
Worst Quarter: -10.31% (3rd Quarter 2008)

Average Annual Total Returns as of 12/31/17

Average Annual Total Returns - AMG Managers Loomis Sayles Bond Fund	1 Year	5 Years	10 Years	Since Inception	[1]	Inception Date
Class N	6.77%	3.27%	5.40%
Class N | Return After Taxes on Distributions	5.14%	1.69%	3.64%
Class N | Return After Taxes on Distributions and Sale of Fund Shares	3.93%	1.84%	3.49%
Class I	6.87%			3.20%		Apr. 01, 2013
Bloomberg Barclays U.S. Govt./Credit Index (reflects no deduction for fees, expenses, or taxes)	4.00%	2.13%	4.08%	2.25%		Apr. 01, 2013

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. After-tax returns are shown for Class N shares only, and after-tax returns for Class I shares will vary.

[1] “Other Expenses” include 0.07% paid by the Fund to AMG Funds LLC (the “Investment Manager”) during the prior fiscal year pursuant to the Fund’s contractual expense limitation agreement, which provides for the recoupment of previously waived management fees and/or reimbursed expenses in certain circumstances when the Fund’s Total Annual Fund Operating Expenses fall below the level in effect when such amounts were waived and/or reimbursed.

[1] Class I and Index performance shown reflects performance since the inception date of the Fund’s Class I shares on April 1, 2013.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	AMG FUNDS III
Prospectus Date	rr_ProspectusDate	May 01, 2018
AMG Managers Loomis Sayles Bond Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	AMG Managers Loomis Sayles Bond Fund
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The AMG Managers Loomis Sayles Bond Fund’s (the “Fund”) investment objective is to achieve a high level of current income.
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.
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)
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 4% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	4.00%
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. 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	The Fund seeks to achieve its investment objective of a high level of current income from a diversified portfolio of fixed-income securities. Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in bonds (debt securities). Under normal market conditions, the Fund invests at least 65% of its total assets in investment grade corporate bonds, mortgage-related and other asset-backed securities and securities issued or guaranteed by the U.S. Government, its agencies or instrumentalities. Investment grade securities are rated at least in the BBB/Baa major rating category by Standard & Poor’s Ratings Services (“S&P”) or Moody’s Investors Service, Inc. (“Moody’s”) (or a similar rating from any nationally recognized statistical rating organization). The Fund may also invest in below investment grade bonds (commonly known as “junk bonds” or “high yield securities”) (those rated below BBB/Baa by S&P or Moody’s). From time to time, the Fund may invest in bonds that are not rated by S&P, Moody’s or a similar nationally recognized statistical rating organization. In the event that the Fund invests in non-rated bonds, the equivalent rating determined by the Fund’s subadviser, Loomis, Sayles & Company, L.P. (the “Subadviser” or “Loomis”), will be used for the purpose of the Fund’s investment policies described above. Debt securities held by the Fund may have any remaining maturity. Occasionally, the Fund may purchase only the interest or principal component of a mortgage-related security. Up to 10% of the total assets of the Fund may be invested in non-U.S. dollar-denominated instruments. Loomis applies fundamental investment research techniques when deciding which bonds to buy or sell. Typically, Loomis invests around three themes encompassing: Generally seeking fixed-income securities of issuers whose credit profiles it believes are improving. Significant use of securities whose price changes may not have a direct correlation with changes in interest rates. Loomis believes positive returns can be generated by having a portion of the Fund's assets invested in this type of security rather than primarily relying on interest rate changes to generate returns. Analysis of different sectors of the global economy and fixed income markets and differences in the yields ("spreads") of various fixed income securities in an effort to find securities that Loomis believes may produce attractive returns for the Fund in comparison to their risk with a preference for discounted securities with a yield advantage to the market average. In deciding which securities to buy and sell, Loomis will consider, among other things, the financial strength of the issuer of the security, current spread/yield levels, Loomis’ expectations regarding general trends in interest rates, and comparisons of the level of risk associated with particular investments in conjunction with Loomis’ expectations concerning the potential return of those investments. Loomis does not manage the Fund to maintain a given average duration, thus giving the Fund the flexibility to invest in securities with any remaining maturity as market conditions change. At times, the Fund’s average duration may be longer than that of the benchmark, so that the Fund is more sensitive to changes in interest rates than the benchmark. At other times the Fund’s average duration may be shorter than that of the benchmark, so that the Fund is less sensitive to changes in interest rates than the benchmark. Loomis may utilize futures contracts in order to adjust the Fund’s duration. Loomis generally prefers securities that seek to protect against calls (early redemption by the issuer).
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 described in alphabetical order and not in the order of importance or potential exposure. Credit and Counterparty Risk—the issuer of bonds or other debt securities or a counterparty to a derivatives contract may be unable or unwilling, or may be perceived as unable or unwilling, to make timely interest, principal or settlement payments or otherwise honor its obligations. Currency Risk—fluctuations in exchange rates may affect the total loss or gain on a non-U.S. dollar investment when converted back to U.S. dollars and exposure to non-U.S. currencies may subject the Fund to the risk that those currencies will decline in value relative to the U.S. dollar. Derivatives Risk—the use of derivatives involves costs, the risk that the value of derivatives may not correlate perfectly with their underlying assets, rates or indices, and the risk of mispricing or improper valuation. The use of derivatives may not succeed for various reasons, and the complexity and rapidly changing structure of derivatives markets may increase the possibility of market losses. Emerging Markets Risk—investments in emerging markets are subject to the general risks of foreign investments, as well as additional risks which can result in greater price volatility. Extension Risk—during periods of rising interest rates, a debtor may pay back a bond or other fixed income security slower than expected or required, and the value of such security may fall. 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 Yield Risk—below investment grade debt securities and unrated securities of similar credit quality (commonly known as “junk bonds” or “high yield securities”) may be subject to greater levels of interest rate, credit, liquidity, and market risk than higher-rated securities. These securities are considered predominately speculative with respect to the issuer’s continuing ability to make principal and interest payments. Inflation/Deflation Risk—Inflation risk is the risk that the value of assets or income from investments will be worth less in the future. Deflation risk is the risk that the prices throughout the economy decline over time—the opposite of inflation. Deflation may have an adverse effect on the creditworthiness of issuers and may make issuer default more likely, which may result in a decline in the value of the Fund’s portfolio. Interest Rate Risk—fixed coupon payments (cash flows) of bonds and debt securities may become less competitive with the market in periods of rising interest rates and cause bond prices to decline. During periods of increasing interest rates, the Fund may experience high levels of volatility and shareholder redemptions, and may have to sell securities at times when it would otherwise not do so, and at unfavorable prices, which could reduce the returns of the Fund. Leverage Risk—borrowing and some derivative investments such as futures and forward commitment transactions, may magnify smaller adverse market movements into relatively larger losses. 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. 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 the Subadviser’s investment techniques and risk analysis will produce the desired result. Market Risk—market prices of investments held by the Fund may fall rapidly or unpredictably due to a variety of factors, including changing economic, political, or market conditions or in response to events that affect particular industries or companies. Prepayment Risk—a debtor may exercise its right to pay back a bond or other debt security earlier than expected or required during periods of decreasing interest rates. Reinvestment Risk—the Fund may have difficulty reinvesting payments from debtors and may receive lower rates than from its original investments.
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 shares of the Fund’s sole share class, which were reclassified and redesignated as 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 800.835.3879.
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	800.835.3879
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 12/31/17 (Class N)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter: 16.27% (2nd Quarter 2009) Worst Quarter: -10.31% (3rd Quarter 2008)
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns as of 12/31/17
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.
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. After-tax returns are shown for Class N shares only, and after-tax returns for Class I shares will vary.
AMG Managers Loomis Sayles Bond Fund | Class N
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.625%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.36%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.99%
1 Year	rr_ExpenseExampleYear01	$ 101
3 Years	rr_ExpenseExampleYear03	315
5 Years	rr_ExpenseExampleYear05	547
10 Years	rr_ExpenseExampleYear10	$ 1,213
2008	rr_AnnualReturn2008	(16.31%)
2009	rr_AnnualReturn2009	31.17%
2010	rr_AnnualReturn2010	10.43%
2011	rr_AnnualReturn2011	6.06%
2012	rr_AnnualReturn2012	12.04%
2013	rr_AnnualReturn2013	1.06%
2014	rr_AnnualReturn2014	5.81%
2015	rr_AnnualReturn2015	(2.15%)
2016	rr_AnnualReturn2016	5.15%
2017	rr_AnnualReturn2017	6.77%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	16.27%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(10.31%)
1 Year	rr_AverageAnnualReturnYear01	6.77%
5 Years	rr_AverageAnnualReturnYear05	3.27%
10 Years	rr_AverageAnnualReturnYear10	5.40%
Since Inception	rr_AverageAnnualReturnSinceInception		[2]
AMG Managers Loomis Sayles Bond Fund | Class I
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.625%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.26%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.89%
1 Year	rr_ExpenseExampleYear01	$ 91
3 Years	rr_ExpenseExampleYear03	284
5 Years	rr_ExpenseExampleYear05	493
10 Years	rr_ExpenseExampleYear10	$ 1,096
1 Year	rr_AverageAnnualReturnYear01	6.87%
5 Years	rr_AverageAnnualReturnYear05
10 Years	rr_AverageAnnualReturnYear10
Since Inception	rr_AverageAnnualReturnSinceInception	3.20%	[2]
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2013
AMG Managers Loomis Sayles Bond Fund | Return After Taxes on Distributions | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	5.14%
5 Years	rr_AverageAnnualReturnYear05	1.69%
10 Years	rr_AverageAnnualReturnYear10	3.64%
Since Inception	rr_AverageAnnualReturnSinceInception		[2]
AMG Managers Loomis Sayles Bond Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.93%
5 Years	rr_AverageAnnualReturnYear05	1.84%
10 Years	rr_AverageAnnualReturnYear10	3.49%
Since Inception	rr_AverageAnnualReturnSinceInception		[2]
AMG Managers Loomis Sayles Bond Fund | Bloomberg Barclays U.S. Govt./Credit Index (reflects no deduction for fees, expenses, or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	4.00%
5 Years	rr_AverageAnnualReturnYear05	2.13%
10 Years	rr_AverageAnnualReturnYear10	4.08%
Since Inception	rr_AverageAnnualReturnSinceInception	2.25%	[2]
Inception Date	rr_AverageAnnualReturnInceptionDate	Apr. 01, 2013

[1] “Other Expenses” include 0.07% paid by the Fund to AMG Funds LLC (the “Investment Manager”) during the prior fiscal year pursuant to the Fund’s contractual expense limitation agreement, which provides for the recoupment of previously waived management fees and/or reimbursed expenses in certain circumstances when the Fund’s Total Annual Fund Operating Expenses fall below the level in effect when such amounts were waived and/or reimbursed.

[2] Class I and Index performance shown reflects performance since the inception date of the Fund’s Class I shares on April 1, 2013.

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AMG Managers Global Income Opportunity Fund

AMG Managers Global Income Opportunity Fund

INVESTMENT OBJECTIVE

The AMG Managers Global Income Opportunity Fund’s (the “Fund”) investment objective is to achieve income and capital appreciation.

FEES AND EXPENSES OF THE FUND

The tables below describe the fees and expenses that you may pay if you buy and hold shares of the Fund.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees	AMG Managers Global Income Opportunity Fund Class N
Redemption/Exchange Fee (as a percentage of the amount redeemed, if applicable, within 60 days of purchase)	1.00%

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses	AMG Managers Global Income Opportunity Fund Class N
Management Fee	0.55%
Distribution and Service (12b-1) Fees	none
Other Expenses	0.84%	[1]
Total Annual Fund Operating Expenses	1.39%
Fee Waiver and Expense Reimbursements	(0.50%)	[2]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	0.89%	[2]

EXPENSE EXAMPLE

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, 2019. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
AMG Managers Global Income Opportunity Fund | Class N | USD ($)	91	391	713	1,625

PORTFOLIO TURNOVER

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 55% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES

The Fund seeks to achieve its investment objective of income and capital appreciation through a portfolio of foreign and domestic fixed income securities. Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in bonds (debt securities). Under normal market conditions, the Fund invests at least 65% of its total assets in investment grade U.S. and foreign corporate bonds and in securities issued or guaranteed by the U.S. and foreign governments, their agencies or instrumentalities, and supranational organizations such as the World Bank. Investment grade securities are rated at least in the BBB/Baa major rating category by Standard & Poor’s Ratings Services (“S&P”) or Moody’s Investor Service, Inc. (“Moody’s”) (or a similar rating from any nationally recognized statistical rating organization). Under normal conditions, the Fund will also invest at least 40% of its net assets, plus the amount of any borrowings for investment purposes, in investments of non-U.S. issuers. The Fund may also invest up to 30% of its total assets in below investment grade bonds (commonly known as “junk bonds” or “high yield securities”) (those rated below BBB/Baa by S&P or Moody’s). In addition, the Fund may invest in privately placed securities, restricted securities, securities that are eligible for resale without registration under the Securities Act of 1933, as amended (the “1933 Act”), pursuant to Rule 144A under the 1933 Act (“Rule 144A Securities”), or other unregistered securities.

The Fund may invest in companies in emerging market countries. Under normal circumstances, the Fund invests no more than 25% of its net assets, plus the amount of any borrowings for investment purposes, in debt securities of companies located in emerging market countries. Emerging market countries include any country determined by the Fund's subadviser, Loomis, Sayles & Company, L.P. (the "Subadviser" or "Loomis") to have an emerging market economy, taking into account a number of factors. These factors may include whether the country has a low-to middle-income economy according to the International Bank for Reconstruction and Development (also known as the World Bank), the country’s foreign currency debt rating, its location and neighboring countries, its political and economic stability and the development of its financial and capital markets. These countries may include those located in Latin America and the Caribbean, Asia, Africa, the former Soviet Union, the Middle East and the developing countries of Europe (primarily Central and Eastern Europe).

From time to time, the Fund may invest in bonds that are not rated by S&P, Moody’s or a similar nationally recognized statistical rating organization. In the event that the Fund invests in non-rated bonds, the equivalent rating determined by Loomis will be used for the purpose of the Fund’s investment policies described above. Debt securities held by the Fund may have any remaining maturity. The Fund may hold instruments denominated in any currency. At least 30% of the total assets of the Fund will be invested in U.S. dollar-denominated instruments.

Loomis applies fundamental investment research techniques when deciding which bonds to buy or sell. Typically, Loomis:

• Primarily selects investments with the goal of enhancing the Fund's overall yield and total return, relative to the Fund's benchmark, the Barclays Global Aggregate Index.

• Uses credit analysis and internally developed investment techniques to evaluate numerous financial criteria relating to debt securities.

• May utilize forward currency contracts in order to adjust the Fund's exposure to foreign currencies.

• Sells a security if it believes the security is overvalued by the market based on its credit, country, or currency.

Loomis does not manage the Fund to maintain a given average duration, thus giving the Fund the flexibility to invest in securities with any remaining maturity as market conditions change. At times, the Fund’s average duration may be longer than that of the benchmark, so that the Fund is more sensitive to changes in interest rates than the benchmark. At other times the Fund’s average duration may be shorter than that of the benchmark, so that the Fund is less sensitive to changes in interest rates than the benchmark. Loomis may utilize futures contracts in order to adjust the Fund’s duration.

Loomis generally prefers securities that seek to protect against calls (early redemption by the issuer).

PRINCIPAL RISKS

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 described in alphabetical order and not in the order of importance or potential exposure.

Credit and Counterparty Risk—the issuer of bonds or other debt securities or a counterparty to a derivatives contract may be unable or unwilling, or may be perceived as unable or unwilling, to make timely interest, principal or settlement payments or otherwise honor its obligations.

Currency Risk—fluctuations in exchange rates may affect the total loss or gain on a non-U.S. dollar investment when converted back to U.S. dollars and exposure to non-U.S. currencies may subject the Fund to the risk that those currencies will decline in value relative to the U.S. dollar.

Derivatives Risk—the use of derivatives involves costs, the risk that the value of derivatives may not correlate perfectly with their underlying assets, rates or indices, and the risk of mispricing or improper valuation. The use of derivatives may not succeed for various reasons, and the complexity and rapidly changing structure of derivatives markets may increase the possibility of market losses.

Emerging Markets Risk—investments in emerging markets are subject to the general risks of foreign investments, as well as additional risks which can result in greater price volatility.

Extension Risk—during periods of rising interest rates, a debtor may pay back a bond or other fixed income security slower than expected or required, and the value of such security may fall.

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.

Hedging Risk—there is no guarantee that hedging strategies will be successful. For example, changes in the value of a hedging transaction may not completely offset changes in the value of the assets and liabilities being hedged. Hedging transactions involve costs and may result in losses.

High Yield Risk—below investment grade debt securities and unrated securities of similar credit quality (commonly known as “junk bonds” or “high yield securities”) may be subject to greater levels of interest rate, credit, liquidity, and market risk than higher-rated securities. These securities are considered predominately speculative with respect to the issuer’s continuing ability to make principal and interest payments.

Inflation/Deflation Risk—Inflation risk is the risk that the value of assets or income from investments will be worth less in the future. Deflation risk is the risk that the prices throughout the economy decline over time—the opposite of inflation. Deflation may have an adverse effect on the creditworthiness of issuers and may make issuer default more likely, which may result in a decline in the value of the Fund’s portfolio.

Interest Rate Risk—fixed coupon payments (cash flows) of bonds and debt securities may become less competitive with the market in periods of rising interest rates and cause bond prices to decline. During periods of increasing interest rates, the Fund may experience high levels of volatility and shareholder redemptions, and may have to sell securities at times when it would otherwise not do so, and at unfavorable prices, which could reduce the returns of the Fund.

Leverage Risk—borrowing and some derivative investments such as futures and forward commitment transactions, may magnify smaller adverse market movements into relatively larger losses.

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.

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 the Subadviser’s investment techniques and risk analysis will produce the desired result.

Market Risk—market prices of investments held by the Fund may fall rapidly or unpredictably due to a variety of factors, including changing economic, political, or market conditions or in response to events that affect particular industries or companies.

Political Risk—changes in the general political and social environment of a country can have substantial effects on the value of investments exposed to that country.

Prepayment Risk—a debtor may exercise its right to pay back a bond or other debt security earlier than expected or required during periods of decreasing interest rates.

Reinvestment Risk—the Fund may have difficulty reinvesting payments from debtors and may receive lower rates than from its original investments.

PERFORMANCE

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 shares of the Fund’s sole share class, which were reclassified and redesignated as Class S shares on October 1, 2016) were renamed Class N shares. To obtain updated performance information please visit www.amgfunds.com or call 800.835.3879.

Calendar Year Total Returns as of 12/31/17 (Class N)

Best Quarter: 14.15% (2nd Quarter 2009)
Worst Quarter: -9.17% (3rd Quarter 2008)

Average Annual Total Returns as of 12/31/17

Average Annual Total Returns - AMG Managers Global Income Opportunity Fund	1 Year	5 Years	10 Years
Class N	10.55%	1.76%	4.12%
Class N | Return After Taxes on Distributions	10.55%	1.21%	2.61%
Class N | Return After Taxes on Distributions and Sale of Fund Shares	5.97%	1.08%	2.56%
Bloomberg Barclays Global Aggregate Index (reflects no deduction for fees, expenses, or taxes)	7.39%	0.79%	3.09%

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.

[1] “Other Expenses” include 0.04% paid by the Fund to AMG Funds LLC (the “Investment Manager”) during the prior fiscal year pursuant to the Fund’s contractual expense limitation agreement, which provides for the recoupment of previously waived management fees and/or reimbursed expenses in certain circumstances when the Fund’s Total Annual Fund Operating Expenses fall below the level in effect when such amounts were waived and/or reimbursed.

[2] The Investment Manager has contractually agreed, through at least May 1, 2019, to waive management fees and/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), brokerage commissions and other transaction costs, acquired fund fees and expenses, and extraordinary expenses) of the Fund to the annual rate of 0.89% of the Fund’s average daily net assets, subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months from the time such amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover from the Fund such fees waived and expenses paid, 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 the contractual expense limitation amount. 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.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	AMG FUNDS III
Prospectus Date	rr_ProspectusDate	May 01, 2018
AMG Managers Global Income Opportunity Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	AMG Managers Global Income Opportunity Fund
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The AMG Managers Global Income Opportunity Fund’s (the “Fund”) investment objective is to achieve income and capital appreciation.
Expense [Heading]	rr_ExpenseHeading	FEES AND EXPENSES OF THE FUND
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The tables below describe the fees and expenses that you may pay if you buy and hold shares of the Fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
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, 2019
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 55% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	55.00%
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, 2019. 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	The Fund seeks to achieve its investment objective of income and capital appreciation through a portfolio of foreign and domestic fixed income securities. Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in bonds (debt securities). Under normal market conditions, the Fund invests at least 65% of its total assets in investment grade U.S. and foreign corporate bonds and in securities issued or guaranteed by the U.S. and foreign governments, their agencies or instrumentalities, and supranational organizations such as the World Bank. Investment grade securities are rated at least in the BBB/Baa major rating category by Standard & Poor’s Ratings Services (“S&P”) or Moody’s Investor Service, Inc. (“Moody’s”) (or a similar rating from any nationally recognized statistical rating organization). Under normal conditions, the Fund will also invest at least 40% of its net assets, plus the amount of any borrowings for investment purposes, in investments of non-U.S. issuers. The Fund may also invest up to 30% of its total assets in below investment grade bonds (commonly known as “junk bonds” or “high yield securities”) (those rated below BBB/Baa by S&P or Moody’s). In addition, the Fund may invest in privately placed securities, restricted securities, securities that are eligible for resale without registration under the Securities Act of 1933, as amended (the “1933 Act”), pursuant to Rule 144A under the 1933 Act (“Rule 144A Securities”), or other unregistered securities. The Fund may invest in companies in emerging market countries. Under normal circumstances, the Fund invests no more than 25% of its net assets, plus the amount of any borrowings for investment purposes, in debt securities of companies located in emerging market countries. Emerging market countries include any country determined by the Fund's subadviser, Loomis, Sayles & Company, L.P. (the "Subadviser" or "Loomis") to have an emerging market economy, taking into account a number of factors. These factors may include whether the country has a low-to middle-income economy according to the International Bank for Reconstruction and Development (also known as the World Bank), the country’s foreign currency debt rating, its location and neighboring countries, its political and economic stability and the development of its financial and capital markets. These countries may include those located in Latin America and the Caribbean, Asia, Africa, the former Soviet Union, the Middle East and the developing countries of Europe (primarily Central and Eastern Europe). From time to time, the Fund may invest in bonds that are not rated by S&P, Moody’s or a similar nationally recognized statistical rating organization. In the event that the Fund invests in non-rated bonds, the equivalent rating determined by Loomis will be used for the purpose of the Fund’s investment policies described above. Debt securities held by the Fund may have any remaining maturity. The Fund may hold instruments denominated in any currency. At least 30% of the total assets of the Fund will be invested in U.S. dollar-denominated instruments. Loomis applies fundamental investment research techniques when deciding which bonds to buy or sell. Typically, Loomis: Primarily selects investments with the goal of enhancing the Fund's overall yield and total return, relative to the Fund's benchmark, the Barclays Global Aggregate Index. Uses credit analysis and internally developed investment techniques to evaluate numerous financial criteria relating to debt securities. May utilize forward currency contracts in order to adjust the Fund's exposure to foreign currencies. Sells a security if it believes the security is overvalued by the market based on its credit, country, or currency. Loomis does not manage the Fund to maintain a given average duration, thus giving the Fund the flexibility to invest in securities with any remaining maturity as market conditions change. At times, the Fund’s average duration may be longer than that of the benchmark, so that the Fund is more sensitive to changes in interest rates than the benchmark. At other times the Fund’s average duration may be shorter than that of the benchmark, so that the Fund is less sensitive to changes in interest rates than the benchmark. Loomis may utilize futures contracts in order to adjust the Fund’s duration. Loomis generally prefers securities that seek to protect against calls (early redemption by the issuer).
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 described in alphabetical order and not in the order of importance or potential exposure. Credit and Counterparty Risk—the issuer of bonds or other debt securities or a counterparty to a derivatives contract may be unable or unwilling, or may be perceived as unable or unwilling, to make timely interest, principal or settlement payments or otherwise honor its obligations. Currency Risk—fluctuations in exchange rates may affect the total loss or gain on a non-U.S. dollar investment when converted back to U.S. dollars and exposure to non-U.S. currencies may subject the Fund to the risk that those currencies will decline in value relative to the U.S. dollar. Derivatives Risk—the use of derivatives involves costs, the risk that the value of derivatives may not correlate perfectly with their underlying assets, rates or indices, and the risk of mispricing or improper valuation. The use of derivatives may not succeed for various reasons, and the complexity and rapidly changing structure of derivatives markets may increase the possibility of market losses. Emerging Markets Risk—investments in emerging markets are subject to the general risks of foreign investments, as well as additional risks which can result in greater price volatility. Extension Risk—during periods of rising interest rates, a debtor may pay back a bond or other fixed income security slower than expected or required, and the value of such security may fall. 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. Hedging Risk—there is no guarantee that hedging strategies will be successful. For example, changes in the value of a hedging transaction may not completely offset changes in the value of the assets and liabilities being hedged. Hedging transactions involve costs and may result in losses. High Yield Risk—below investment grade debt securities and unrated securities of similar credit quality (commonly known as “junk bonds” or “high yield securities”) may be subject to greater levels of interest rate, credit, liquidity, and market risk than higher-rated securities. These securities are considered predominately speculative with respect to the issuer’s continuing ability to make principal and interest payments. Inflation/Deflation Risk—Inflation risk is the risk that the value of assets or income from investments will be worth less in the future. Deflation risk is the risk that the prices throughout the economy decline over time—the opposite of inflation. Deflation may have an adverse effect on the creditworthiness of issuers and may make issuer default more likely, which may result in a decline in the value of the Fund’s portfolio. Interest Rate Risk—fixed coupon payments (cash flows) of bonds and debt securities may become less competitive with the market in periods of rising interest rates and cause bond prices to decline. During periods of increasing interest rates, the Fund may experience high levels of volatility and shareholder redemptions, and may have to sell securities at times when it would otherwise not do so, and at unfavorable prices, which could reduce the returns of the Fund. Leverage Risk—borrowing and some derivative investments such as futures and forward commitment transactions, may magnify smaller adverse market movements into relatively larger losses. 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. 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 the Subadviser’s investment techniques and risk analysis will produce the desired result. Market Risk—market prices of investments held by the Fund may fall rapidly or unpredictably due to a variety of factors, including changing economic, political, or market conditions or in response to events that affect particular industries or companies. Political Risk—changes in the general political and social environment of a country can have substantial effects on the value of investments exposed to that country. Prepayment Risk—a debtor may exercise its right to pay back a bond or other debt security earlier than expected or required during periods of decreasing interest rates. Reinvestment Risk—the Fund may have difficulty reinvesting payments from debtors and may receive lower rates than from its original investments.
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 shares of the Fund’s sole share class, which were reclassified and redesignated as Class S shares on October 1, 2016) were renamed Class N shares. To obtain updated performance information please visit www.amgfunds.com or call 800.835.3879.
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	800.835.3879
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 12/31/17 (Class N)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter: 14.15% (2nd Quarter 2009) Worst Quarter: -9.17% (3rd Quarter 2008)
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns as of 12/31/17
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.
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.
AMG Managers Global Income Opportunity Fund | Class N
Risk/Return:	rr_RiskReturnAbstract
Redemption/Exchange Fee (as a percentage of the amount redeemed, if applicable, within 60 days of purchase)	rr_RedemptionFeeOverRedemption	1.00%
Redemption/Exchange Fee (as a percentage of the amount redeemed, if applicable, within 60 days of purchase)	rr_ExchangeFeeOverRedemption	1.00%
Management Fee	rr_ManagementFeesOverAssets	0.55%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.84%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.39%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.50%)	[2]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	0.89%	[2]
1 Year	rr_ExpenseExampleYear01	$ 91
3 Years	rr_ExpenseExampleYear03	391
5 Years	rr_ExpenseExampleYear05	713
10 Years	rr_ExpenseExampleYear10	$ 1,625
2008	rr_AnnualReturn2008	(10.01%)
2009	rr_AnnualReturn2009	24.26%
2010	rr_AnnualReturn2010	7.27%
2011	rr_AnnualReturn2011	3.39%
2012	rr_AnnualReturn2012	10.63%
2013	rr_AnnualReturn2013	(1.40%)
2014	rr_AnnualReturn2014	1.84%
2015	rr_AnnualReturn2015	(6.22%)
2016	rr_AnnualReturn2016	4.79%
2017	rr_AnnualReturn2017	10.55%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	14.15%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(9.17%)
1 Year	rr_AverageAnnualReturnYear01	10.55%
5 Years	rr_AverageAnnualReturnYear05	1.76%
10 Years	rr_AverageAnnualReturnYear10	4.12%
AMG Managers Global Income Opportunity Fund | Return After Taxes on Distributions | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	10.55%
5 Years	rr_AverageAnnualReturnYear05	1.21%
10 Years	rr_AverageAnnualReturnYear10	2.61%
AMG Managers Global Income Opportunity Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	5.97%
5 Years	rr_AverageAnnualReturnYear05	1.08%
10 Years	rr_AverageAnnualReturnYear10	2.56%
AMG Managers Global Income Opportunity Fund | Bloomberg Barclays Global Aggregate Index (reflects no deduction for fees, expenses, or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	7.39%
5 Years	rr_AverageAnnualReturnYear05	0.79%
10 Years	rr_AverageAnnualReturnYear10	3.09%

[1] “Other Expenses” include 0.04% paid by the Fund to AMG Funds LLC (the “Investment Manager”) during the prior fiscal year pursuant to the Fund’s contractual expense limitation agreement, which provides for the recoupment of previously waived management fees and/or reimbursed expenses in certain circumstances when the Fund’s Total Annual Fund Operating Expenses fall below the level in effect when such amounts were waived and/or reimbursed.

[2] The Investment Manager has contractually agreed, through at least May 1, 2019, to waive management fees and/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), brokerage commissions and other transaction costs, acquired fund fees and expenses, and extraordinary expenses) of the Fund to the annual rate of 0.89% of the Fund’s average daily net assets, subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months from the time such amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover from the Fund such fees waived and expenses paid, 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 the contractual expense limitation amount. 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.

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Total

Class R Shares | AMG Managers Special Equity Fund

AMG FUNDS III

AMG MANAGERS SPECIAL EQUITY FUND

Supplement to Prospectus dated May 1, 2018

Multiple Classes - Supplemental Information

Multiple Class Information

AMG Managers Special Equity Fund (the “Fund”) currently offers Class N shares and Class I shares. In addition to the classes of shares of the Fund currently being offered, the Fund may from time to time issue R Shares.

Each class of shares may be subject to different distribution and shareholder servicing arrangements, which may result in differences in the expenses borne by the shareholders of each class and the returns realized by such shareholders. In addition, each class of shares of the Fund is subject to a different minimum initial investment requirement, as described in greater detail below.

R Shares are available for purchase exclusively through financial intermediaries and are also subject to a Distribution and Service Plan (12b-1 Plan) adopted by the Board of Trustees of AMG Funds III (the “Trust”).

As a result of these differences in the distribution and servicing arrangements, R Shares can be expected to bear higher expenses than Class N and Class I shares and to experience lower returns. In all other material respects, the shares of each class are the same, representing a proportional interest in the Fund.

Performance

Because R Shares of the Fund have not commenced operations, the class has no performance history.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

	R Shares
Management Fee		0.90	%
Distribution (12b-1) Fees		0.50	%
Other Expenses		0.26	%
Total Annual Fund Operating Expenses		1.66	%
Fee Waiver and Expense Reimbursements1		(0.05	)%
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements1		1.61	%

¹  AMG Funds LLC (the “Investment Manager”) has contractually agreed, through at least May 1, 2019, to waive management fees and/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), shareholder servicing fees, brokerage commissions and other transaction costs, 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, subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months from the time such amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover from the Fund such fees waived and expenses paid, 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 the contractual expense limitation amount. 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.

Example

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, 2019. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:

	1 Year		3 Years		5 Years		10 Years
R Shares	$	164	$	519	$	897	$	1,961

The Example should not be considered a representation of past or future expenses, as actual expenses may be greater or lower than those shown. 

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

AMG FUNDS III

Prospectus Date

rr_ProspectusDate

May 01, 2018

Class R Shares | AMG Managers Special Equity Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

afiii_SupplementTextBlock

AMG FUNDS III

AMG MANAGERS SPECIAL EQUITY FUND

Supplement to Prospectus dated May 1, 2018

Multiple Classes - Supplemental Information

Multiple Class Information

AMG Managers Special Equity Fund (the “Fund”) currently offers Class N shares and Class I shares. In addition to the classes of shares of the Fund currently being offered, the Fund may from time to time issue R Shares.

Each class of shares may be subject to different distribution and shareholder servicing arrangements, which may result in differences in the expenses borne by the shareholders of each class and the returns realized by such shareholders. In addition, each class of shares of the Fund is subject to a different minimum initial investment requirement, as described in greater detail below.

R Shares are available for purchase exclusively through financial intermediaries and are also subject to a Distribution and Service Plan (12b-1 Plan) adopted by the Board of Trustees of AMG Funds III (the “Trust”).

As a result of these differences in the distribution and servicing arrangements, R Shares can be expected to bear higher expenses than Class N and Class I shares and to experience lower returns. In all other material respects, the shares of each class are the same, representing a proportional interest in the Fund.

Performance

Because R Shares of the Fund have not commenced operations, the class has no performance history.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

	R Shares
Management Fee		0.90	%
Distribution (12b-1) Fees		0.50	%
Other Expenses		0.26	%
Total Annual Fund Operating Expenses		1.66	%
Fee Waiver and Expense Reimbursements1		(0.05	)%
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements1		1.61	%

¹  AMG Funds LLC (the “Investment Manager”) has contractually agreed, through at least May 1, 2019, to waive management fees and/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), shareholder servicing fees, brokerage commissions and other transaction costs, 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, subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months from the time such amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover from the Fund such fees waived and expenses paid, 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 the contractual expense limitation amount. 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.

Example

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, 2019. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:

	1 Year		3 Years		5 Years		10 Years
R Shares	$	164	$	519	$	897	$	1,961

The Example should not be considered a representation of past or future expenses, as actual expenses may be greater or lower than those shown. 

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, 2019

Expense Example [Heading]

rr_ExpenseExampleHeading

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, 2019. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:

Expense Example Closing [Text Block]

rr_ExpenseExampleClosingTextBlock

The Example should not be considered a representation of past or future expenses, as actual expenses may be greater or lower than those shown.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

Because R Shares of the Fund have not commenced operations, the class has no performance history.

Performance One Year or Less [Text]

rr_PerformanceOneYearOrLess

Because R Shares of the Fund have not commenced operations, the class has no performance history.

Class R Shares | AMG Managers Special Equity Fund | R Shares

Risk/Return:

rr_RiskReturnAbstract

Management Fee

rr_ManagementFeesOverAssets

0.90%

Distribution (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.50%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.66%

Fee Waiver and Expense Reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.05%)

^[1]

Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements

rr_NetExpensesOverAssets

1.61%

^[1]

1 Year

rr_ExpenseExampleYear01

$ 164

3 Years

rr_ExpenseExampleYear03

519

5 Years

rr_ExpenseExampleYear05

897

10 Years

rr_ExpenseExampleYear10

$ 1,961

[1] AMG Funds LLC (the "Investment Manager") has contractually agreed, through at least May 1, 2019, to waive management fees and/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), shareholder servicing fees, brokerage commissions and other transaction costs, 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, subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months from the time such amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover from the Fund such fees waived and expenses paid, 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 the contractual expense limitation amount. 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.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	AMG FUNDS III
Prospectus Date	rr_ProspectusDate	May 01, 2018
Document Creation Date	dei_DocumentCreationDate	Apr. 30, 2018