Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	AMG FUNDS III
Prospectus Date	rr_ProspectusDate	Oct. 01, 2016
AMG Managers Cadence Capital Appreciation Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	AMG Managers Cadence Capital Appreciation Fund
Objective [Heading]	rr_ObjectiveHeading	INVESTMENT OBJECTIVE
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The AMG Managers Cadence Capital Appreciation Fund’s (the “Fund”) investment objective is to seek growth of capital.
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	October 1, 2017
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 22% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	22.00%
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Expense information has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	EXPENSE EXAMPLE
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example will help you compare the cost of investing in the Fund to the cost of investing in other mutual funds. The Example makes certain assumptions. It assumes that you invest $10,000 as an initial investment in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. It also assumes that your investment has a 5% total return each year and the Fund’s operating expenses remain the same. The Example includes the Fund’s contractual expense limitation through October 1, 2017. 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 circumstances, the Fund seeks to achieve its investment objective by investing at least 65% of its net assets in common stocks of U.S. companies with market capitalizations of $3 billion or more that have improving fundamentals (based on growth criteria) and whose stock the portfolio management team believes to be reasonably valued by the market (based on value criteria). The Fund primarily invests in common stocks of large capitalization companies. Cadence Capital Management LLC (“Cadence” or the “Subadvisor”) employs a “growth-at-a-reasonable-price” (“GARP”) investment philosophy, allowing the team to invest in companies that exhibit both growth and value characteristics. In making investment decisions for the Fund, the Subadvisor considers companies in the Russell 1000® Index and the S&P 500 Index. The team ranks the stocks in this universe based on a series of growth criteria, such as the change in consensus earnings estimates over time, the company’s history in meeting earnings targets, earnings quality, and improvements in return on capital, and a series of value criteria, such as price-to-earnings ratios and free cash flow relative to enterprise value. The Subadvisor then subjects the most attractively ranked stocks in the universe to an analysis of company factors, such as strength of management, competitive industry position and business prospects, and financial statement data, such as earnings, cash flows and profitability. The team may emphasize one or more of these growth or value criteria, or use other criteria, when selecting investments for the Fund. The Fund may invest a portion of its assets in real estate investment trusts (“REITs”). Generally, the Fund will hold between approximately 70-95 securities.
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. 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. Large-Capitalization Stock Risk—the stocks of large-capitalization companies are generally more mature and may not be able to reach the same levels of growth as the stocks of small- or mid-capitalization companies. 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 Subadvisor’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. Mid-Capitalization Stock Risk—the stocks of mid-capitalization companies often have greater price volatility, lower trading volume, and less liquidity than the stocks of larger, more established companies. Real Estate Industry Risk—investments in the Fund may be subject to many of the same risks as a direct investment in real estate; in addition, equity REITs may be affected by changes in the value of their underlying properties. 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 sector currently, and may in the future, comprise a significant portion of the Fund’s portfolio. The information technology industries may be affected by technological obsolescence, short product cycles, falling prices and profits, competitive pressures and general market conditions.
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. The performance information for the Fund’s Class N shares (formerly Investor Class shares of the Fund, which were renamed Class N shares on October 1, 2016 (formerly Class A shares of the Fund, which were renamed Investor Class shares on December 1, 2012)) for periods prior to December 1, 2012, does not reflect the impact of the front end and deferred sales charges (loads) that were in effect until December 1, 2012. Effective October 1, 2016, outstanding Service Class shares of the Fund (formerly Administrative Class shares of the Fund, which were renamed Service Class shares on December 1, 2012) were renamed Class I shares of the Fund and outstanding Institutional Class shares of the Fund were renamed Class Z shares. As always, past performance of the Fund (before and after taxes) is not an indication of how the Fund will perform in the future. To obtain updated performance information for the Fund please visit www.amgfunds.com or call 800.835.3879. The performance information shown in the bar chart is that of the Fund’s Class Z shares, and the performance information shown in the bar chart and table includes historical performance of the Fund for periods prior to September 27, 2010, which was the date the Fund was reorganized from the Allianz CCM Capital Appreciation Fund to the Fund.
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 12/31/15 (Class Z)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter: 15.55% (1st Quarter 2012) Worst Quarter: -22.41% (4th Quarter 2008) Year-to-Date (as of 6/30/16): -0.45%
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns as of 12/31/15
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 Z shares only. After-tax returns for Class I and Class N 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 Z shares only. After-tax returns for Class I and Class N shares will vary.
AMG Managers Cadence Capital Appreciation Fund | Class N
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.45%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.35%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.05%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[2]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.02%	[2]
1 Year	rr_ExpenseExampleYear01	$ 104
3 Years	rr_ExpenseExampleYear03	331
5 Years	rr_ExpenseExampleYear05	576
10 Years	rr_ExpenseExampleYear10	$ 1,280
1 Year	rr_AverageAnnualReturnYear01	3.42%
5 Years	rr_AverageAnnualReturnYear05	9.74%
10 Years	rr_AverageAnnualReturnYear10	4.88%
AMG Managers Cadence Capital Appreciation Fund | Class I
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.45%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.45%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.90%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[2]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	0.87%	[2]
1 Year	rr_ExpenseExampleYear01	$ 89
3 Years	rr_ExpenseExampleYear03	284
5 Years	rr_ExpenseExampleYear05	496
10 Years	rr_ExpenseExampleYear10	$ 1,105
1 Year	rr_AverageAnnualReturnYear01	3.55%
5 Years	rr_AverageAnnualReturnYear05	9.89%
10 Years	rr_AverageAnnualReturnYear10	5.03%
AMG Managers Cadence Capital Appreciation Fund | Class Z
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.45%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.30%	[1]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.75%
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[2]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	0.72%	[2]
1 Year	rr_ExpenseExampleYear01	$ 74
3 Years	rr_ExpenseExampleYear03	237
5 Years	rr_ExpenseExampleYear05	414
10 Years	rr_ExpenseExampleYear10	$ 928
2006	rr_AnnualReturn2006	7.09%
2007	rr_AnnualReturn2007	17.55%
2008	rr_AnnualReturn2008	(42.54%)
2009	rr_AnnualReturn2009	22.92%
2010	rr_AnnualReturn2010	16.07%
2011	rr_AnnualReturn2011	(2.70%)
2012	rr_AnnualReturn2012	10.60%
2013	rr_AnnualReturn2013	30.99%
2014	rr_AnnualReturn2014	10.90%
2015	rr_AnnualReturn2015	3.77%
Year to Date Return, Label	rr_YearToDateReturnLabel	Year-to-Date
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Jun. 30, 2016
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	(0.45%)
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2012
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	15.55%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(22.41%)
1 Year	rr_AverageAnnualReturnYear01	3.77%
5 Years	rr_AverageAnnualReturnYear05	10.16%
10 Years	rr_AverageAnnualReturnYear10	5.29%
AMG Managers Cadence Capital Appreciation Fund | Return After Taxes on Distributions | Class Z
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.56%
5 Years	rr_AverageAnnualReturnYear05	9.97%
10 Years	rr_AverageAnnualReturnYear10	4.84%
AMG Managers Cadence Capital Appreciation Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class Z
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	2.32%
5 Years	rr_AverageAnnualReturnYear05	8.07%
10 Years	rr_AverageAnnualReturnYear10	4.20%
AMG Managers Cadence Capital Appreciation Fund | Russell 1000® Growth Index (reflects no deduction for fees, expenses, or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	5.67%
5 Years	rr_AverageAnnualReturnYear05	13.53%
10 Years	rr_AverageAnnualReturnYear10	8.53%

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

[2] AMG Funds LLC (the “Investment Manager”) has contractually agreed, through at least October 1, 2017, 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, distribution and service (12b-1) fees, brokerage commissions and other transaction costs, acquired fund fees and expenses, and extraordinary expenses) of the Fund to the annual rate of 0.72% 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, the Investment Manager may recover from the Fund fees waived and expenses paid pursuant to this contractual agreement, 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.