Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	AMG Funds IV
Prospectus Date	rr_ProspectusDate	Mar. 01, 2019
AMG River Road Small-Mid Cap Value Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	<b>AMG River Road Small-Mid Cap Value Fund</b>
Objective [Heading]	rr_ObjectiveHeading	<b>INVESTMENT OBJECTIVE</b>
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Fund seeks to provide long-term capital appreciation.
Expense [Heading]	rr_ExpenseHeading	<b>FEES AND EXPENSES OF THE FUND</b>
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes the fees and expenses that you may pay if you buy and hold shares of the Fund. If you transact in Class I shares of the Fund through a financial intermediary, you may be required to pay a commission to the financial intermediary for effecting such transactions. Such commissions are charged by the financial intermediary and are not reflected in the table or Expense Example below.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses<br/>(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	March 1, 2020
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	<b>PORTFOLIO TURNOVER</b>
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 46% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	46.00%
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Expense information has been restated to reflect current fees.
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	The Total Annual Fund Operating Expenses and Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements do not correlate to the ratios of expenses to average net assets in the Financial Highlights section of this Prospectus, which reflect only the operating expenses of the Fund and do not include fees and expenses of any acquired fund.
Expense Example [Heading]	rr_ExpenseExampleHeading	<b>EXPENSE EXAMPLE</b>
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 March 1, 2020. Although your actual costs may be higher or lower, based on the above assumptions, your costs would be:
Strategy [Heading]	rr_StrategyHeading	<b>PRINCIPAL INVESTMENT STRATEGIES</b>
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its net assets, plus the amount of any borrowings for investment purposes, in equity securities of small- and mid-capitalization companies that River Road Asset Management, LLC, the subadviser to the Fund (“River Road” or the “Subadviser”), believes are undervalued. The Fund currently considers small- and mid-capitalization companies to be those with market capitalizations at the time of acquisition within the range of capitalizations of companies in the Russell 2500TM Index (between $159.2 million and $12.0 billion as of May 11, 2018, the date of the latest reconstitution of the Index (implemented by the Index June 22, 2018)). This capitalization range will change over time. The Fund may continue to hold securities of a portfolio company that subsequently drops below or appreciates above this capitalization threshold. Because of this, the Fund may have less than 80% of its net assets in securities of small- and mid-cap companies at any given time. Value investing involves buying stocks that the Subadviser believes are out of favor and/or undervalued in comparison to their peers or their prospects for growth. The Fund may also invest in common stock of companies with market capitalizations that exceed that of the Russell 2500TM Index at the time of acquisition, real estate investment trusts (“REITs”), convertible preferred stocks, investment companies (such as exchange-traded funds (“ETFs”), open-end funds and closed-end funds) and foreign securities (directly and through depositary receipts). The Subadviser’s investment philosophy is based upon its proprietary Absolute Value® approach, which seeks to provide attractive, sustainable, low volatility returns over the long term, while reducing downside portfolio risk. The Subadviser uses systematic and dynamic proprietary research to analyze companies based on investment criteria such as one or more of the following: Security price that is at a discount to assessed valuation as determined by the Subadviser’s unique and proprietary Absolute Value® approach Attractive business model Shareholder-oriented management Financial strength Undiscovered, underfollowed or misunderstood companies To seek to manage risk, the Subadviser employs a strategy of balanced diversification, and adheres to a structured sell discipline.
Risk [Heading]	rr_RiskHeading	<b>PRINCIPAL RISKS</b>
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. Convertible Securities Risk— convertible preferred stocks, which are convertible into shares of the issuer’s common stock and pay regular dividends, and convertible debt securities, which are convertible into shares of the issuer’s common stock and bear interest, are subject to the risks of equity securities and fixed income securities. The lower the conversion premium, the more likely the price of the convertible security will follow the price of the underlying common stock. Conversely, higher premium convertible securities are more likely to exhibit the behavior of bonds because the likelihood of conversion is lower, which may cause their prices to fall as interest rates rise. There is the risk that the issuer of convertible preferred stock will not be able to make dividend payments or that the issuer of a convertible bond will not be able to make principal and/or interest payments. 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. 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. Investment Company Risk—the risks of investing in other investment companies, including ETFs, open-end funds and closed-end funds, typically reflect the risks of the types of securities in which those investment companies invest. The market value returns of closed-end funds and ETFs may lag their returns at net asset value. When the Fund invests in another investment company, shareholders of the Fund bear their proportionate share of the other investment company’s fees and expenses as well as their share of the Fund’s fees and expenses. 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 economic, political, or market conditions or in response to events that affect particular industries or 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. The stock prices of companies in the real estate industry, including REITs, are typically sensitive to changes in real estate values, property taxes, interest rates, cash flow of underlying real estate assets, occupancy rates, government regulations affecting zoning, land use, and rents, as well as the management skill and creditworthiness of the issuer. REITs also depend generally on their ability to generate cash flow to make distributions to shareholders or unitholders and are subject to the risk of failing to qualify for favorable tax treatment under the Internal Revenue Code of 1986, as amended (the “Internal Revenue Code”). 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 consumer discretionary and industrials sectors may comprise a significant portion of the Fund's portfolio.  The consumer discretionary sector may be affected by the performance of the overall economy, consumer confidence and spending, changes in demographics and consumer tastes, interest rates, and competitive pressures. The industrials sector may be affected by general economic trends, including employment, economic growth and interest rates, changes in consumer confidence and spending, government regulation, commodity prices and competitive pressures. 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. Value Stock Risk—value stocks may perform differently from the market as a whole and may be undervalued by the market for a long period of time.
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	<b>PERFORMANCE</b>
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 two broad-based securities market indices. 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 please visit www.amgfunds.com or call 800.548.4539.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The following performance information illustrates the risks of investing in the Fund by showing changes in the Fund's performance from year to year and by showing how the Fund's performance compares to that of two broad-based securities market indices.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800.548.4539
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	www.amgfunds.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As always, past performance of the Fund (before and after taxes) is not an indication of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Calendar Year Total Returns as of 12/31/18 (Class N)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	Best Quarter: 14.59% (4th Quarter 2011) Worst Quarter: -16.57% (4th Quarter 2018)
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns as of 12/31/18
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns are not relevant to investors who hold their Fund shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns are shown for Class N shares only, and after-tax returns for Class I and Class Z shares will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns are not relevant to investors who hold their Fund shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”). After-tax returns are shown for Class N shares only, and after-tax returns for Class I and Class Z shares will vary.
AMG River Road Small-Mid Cap Value Fund | Class N
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.75%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.23%
Other Expenses	rr_OtherExpensesOverAssets	0.38%	[1]
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.17%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.53%	[2]
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[3]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.50%	[2],[3]
1 Year	rr_ExpenseExampleYear01	$ 153
3 Years	rr_ExpenseExampleYear03	480
5 Years	rr_ExpenseExampleYear05	831
10 Years	rr_ExpenseExampleYear10	$ 1,821
2009	rr_AnnualReturn2009	19.65%
2010	rr_AnnualReturn2010	21.65%
2011	rr_AnnualReturn2011	(0.12%)
2012	rr_AnnualReturn2012	15.58%
2013	rr_AnnualReturn2013	27.74%
2014	rr_AnnualReturn2014	(0.33%)
2015	rr_AnnualReturn2015	(2.76%)
2016	rr_AnnualReturn2016	27.13%
2017	rr_AnnualReturn2017	15.35%
2018	rr_AnnualReturn2018	(6.44%)
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Best Quarter:
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Dec. 31, 2011
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	14.59%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Worst Quarter:
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2018
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(16.57%)
1 Year	rr_AverageAnnualReturnYear01	(6.44%)
5 Years	rr_AverageAnnualReturnYear05	5.86%
10 Years	rr_AverageAnnualReturnYear10	11.06%
Since Inception	rr_AverageAnnualReturnSinceInception		[4]
AMG River Road Small-Mid Cap Value Fund | Class I
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.75%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.37%	[1]
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.17%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.29%	[2]
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[3]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.26%	[2],[3]
1 Year	rr_ExpenseExampleYear01	$ 128
3 Years	rr_ExpenseExampleYear03	406
5 Years	rr_ExpenseExampleYear05	705
10 Years	rr_ExpenseExampleYear10	$ 1,554
1 Year	rr_AverageAnnualReturnYear01	(6.18%)
5 Years	rr_AverageAnnualReturnYear05	6.15%
10 Years	rr_AverageAnnualReturnYear10	11.35%
Since Inception	rr_AverageAnnualReturnSinceInception		[4]
AMG River Road Small-Mid Cap Value Fund | Class Z
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.75%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.32%	[1]
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.17%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.24%	[2]
Fee Waiver and Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[3]
Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements	rr_NetExpensesOverAssets	1.21%	[2],[3]
1 Year	rr_ExpenseExampleYear01	$ 123
3 Years	rr_ExpenseExampleYear03	390
5 Years	rr_ExpenseExampleYear05	678
10 Years	rr_ExpenseExampleYear10	$ 1,497
1 Year	rr_AverageAnnualReturnYear01	(6.20%)
5 Years	rr_AverageAnnualReturnYear05
10 Years	rr_AverageAnnualReturnYear10
Since Inception	rr_AverageAnnualReturnSinceInception	(3.65%)	[4]
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 29, 2017
AMG River Road Small-Mid Cap Value Fund | Return After Taxes on Distributions | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(8.51%)
5 Years	rr_AverageAnnualReturnYear05	2.55%
10 Years	rr_AverageAnnualReturnYear10	8.41%
Since Inception	rr_AverageAnnualReturnSinceInception		[4]
AMG River Road Small-Mid Cap Value Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class N
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.85%)
5 Years	rr_AverageAnnualReturnYear05	4.00%
10 Years	rr_AverageAnnualReturnYear10	8.75%
Since Inception	rr_AverageAnnualReturnSinceInception		[4]
AMG River Road Small-Mid Cap Value Fund | Russell 2500® Value Index (reflects no deduction for fees, expenses, or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(12.36%)
5 Years	rr_AverageAnnualReturnYear05	4.16%
10 Years	rr_AverageAnnualReturnYear10	11.62%
Since Inception	rr_AverageAnnualReturnSinceInception	(6.99%)	[4]
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 29, 2017
AMG River Road Small-Mid Cap Value Fund | Russell 2000® Value Index (reflects no deduction for fees, expenses, or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(12.86%)
5 Years	rr_AverageAnnualReturnYear05	3.61%
10 Years	rr_AverageAnnualReturnYear10	10.40%
Since Inception	rr_AverageAnnualReturnSinceInception	(8.99%)	[4]
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 29, 2017

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

[2] The Total Annual Fund Operating Expenses and Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements do not correlate to the ratios of expenses to average net assets in the Financial Highlights section of this Prospectus, which reflect only the operating expenses of the Fund and do not include fees and expenses of any acquired fund.

[3] AMG Funds LLC (the “Investment Manager”) has contractually agreed, through at least March 1, 2020, to waive management fees and/or pay or reimburse the Fund’s expenses in order to limit Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements (exclusive of taxes, interest (including interest incurred in connection with bank and custody overdrafts and in connection with securities sold short), shareholder servicing fees, distribution and service (12b-1) fees, brokerage commissions and other transaction costs, dividends payable with respect to securities sold short, acquired fund fees and expenses, and extraordinary expenses) of the Fund to the annual rate of 1.04% of the Fund’s average daily net assets (this annual rate or such other annual rate that may be in effect from time to time, the “Expense Cap”), subject to later reimbursement by the Fund in certain circumstances. In general, for a period of up to 36 months after the date any amounts are paid, waived or reimbursed by the Investment Manager, the Investment Manager may recover such amounts from the Fund, provided that such repayment would not cause the Fund’s Total Annual Fund Operating Expenses After Fee Waiver and Expense Reimbursements (exclusive of the items noted in the parenthetical above) to exceed either (i) the Expense Cap in effect at the time such amounts were paid, waived or reimbursed, or (ii) the Expense Cap in effect at the time of such repayment by the Fund. The contractual expense limitation may only be terminated in the event the Investment Manager or a successor ceases to be the investment manager of the Fund or a successor fund, by mutual agreement between the Investment Manager and the AMG Funds IV 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.

[4] Class Z and Index peformance shown reflects performance since the inception date of the Fund’s Class Z shares on September 29, 2017.