Label	Element	Value
BARRETT OPPORTUNITY FUND, INC.
Prospectus [Line Items]	rr_ProspectusLineItems
Risk/Return [Heading]	rr_RiskReturnHeading	Barrett Opportunity Fund, Inc.
Objective [Heading]	rr_ObjectiveHeading	Investment Objectives
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	Barrett Opportunity Fund, Inc. (the “Fund”) seeks to achieve above average long-term capital appreciation.
Objective, Secondary [Text Block]	rr_ObjectiveSecondaryTextBlock	Current income is a secondary objective.
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 hold and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the table and example below.
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)
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 may generate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs and potentially higher taxes, 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 3% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	3.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes: •You invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods •Your investment has a 5% return each year and the Fund’s operating expenses remain the same You would incur these hypothetical expenses whether or not you redeem your investment at the end of the given period.
Expense Example by, Year, Caption [Text]	rr_ExpenseExampleByYearCaption	Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund’s investment objectives are not fundamental and may be changed by the Board of Directors upon 60 days’ written notice to holders of the Fund’s common shares. The Fund invests primarily in common stocks and securities convertible into or exchangeable for common stock such as convertible preferred stock or convertible debt securities. The Fund may also invest without limit in fixed-income securities (including up to 5% of its net assets in fixed-income securities that are high yield, lower quality securities rated by a rating organization below its top four long-term rating categories (i.e., below investment grade securities, commonly referred to as “junk bonds”)) or unrated securities determined by the Adviser (as defined below) to be of equivalent quality. The Fund is non-diversified.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. There is no assurance that the Fund will meet its investment objectives. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of certain risks of investing in the Fund. Stock market and equity securities risk. The securities markets are volatile and the market prices of the Fund’s securities may decline generally. Securities fluctuate in price based on changes in a company’s financial condition and overall market and economic conditions, which are difficult to predict accurately. If the market prices of the securities owned by the Fund fall, the value of your investment in the Fund will decline. Issuer risk. The value of a security can go up or down more than the market as a whole and can perform differently from the value of the market as a whole, often due to events such as (but not limited to) disappointing earnings reports by the issuer, unsuccessful products or services, loss of major customers, major litigation against the issuer or changes in government regulations affecting the issuer or the competitive environment. The Fund may experience a substantial or complete loss on an individual security. Historically, the prices of securities of small- and medium-capitalization companies have generally gone up or down more than those of large capitalization companies because, among other things, small- and medium-capitalization companies may be more sensitive to changing economic conditions and tend to be less established than large capitalization companies, although even large capitalization companies may fall out of favor with investors. Foreign investments risk. The Fund’s investments in securities of foreign issuers involve greater risk than investments in securities of U.S. issuers. Foreign countries in which the Fund may invest may have markets that are less liquid and more volatile than U.S. markets and may suffer from political or economic instability. In some foreign countries, less information is available about issuers and markets because of less rigorous accounting, legal and regulatory standards than in the United States. Currency fluctuations could erase investment gains or add to investment losses. Income earned on foreign securities may be subject to foreign withholding taxes. Portfolio selection risk. The value of your investment may decrease if the portfolio managers’ judgment about the attractiveness, value or market trends affecting a particular security, industry or sector or about market movements is incorrect. Value investing risk. The value approach to investing involves the risk that stocks may remain undervalued. Value stocks may underperform the overall equity market. Although the Fund will not concentrate its investments in any one industry or industry group, it may, like many value funds, weight its investments toward certain industries, thus increasing its exposure to factors adversely affecting issuers within those industries. Accordingly, the Fund’s performance may be disproportionately affected by the poor performance of a certain industry. Recent market events risk. U.S. and international markets have experienced significant periods of volatility in recent years due to a number of economic, political and global macro factors including the impact of the coronavirus (COVID-19) as a global pandemic which has resulted in public health issues, growth concerns in the U.S. and overseas, layoffs, rising unemployment and reduced consumer spending. The effects of COVID-19 may lead to a substantial economic downturn or recession in the U.S. and global economies, the recovery from which is uncertain and may last for an extended period of time. Non-diversification risk. The Fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. To the extent the Fund invests its assets in fewer issuers, the Fund will be more susceptible to negative events affecting those issuers. Cybersecurity risk. With the increased use of technologies such as the Internet to conduct business, the Fund is susceptible to operational, information security, and related risks. Cyber incidents affecting the Fund or its service providers may cause disruptions and impact business operations, potentially resulting in financial losses, interference with the Fund’s ability to calculate its NAV, impediments to trading, the inability of shareholders to transact business, violations of applicable privacy and other laws, regulatory fines, penalties, reputational damage, reimbursement or other compensation costs, or additional compliance costs. Risk of increase in expenses. Your actual costs of investing in the Fund may be higher than the expenses shown in “Annual Fund Operating Expenses” under “Fees and Expenses of the Fund” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease. Net assets are more likely to decrease and fund expense ratios are more likely to increase when markets are volatile. Additionally, a higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. Net unrealized appreciation. The Fund currently has a substantial amount of net unrealized appreciation. At November 30, 2021, the amount of the net unrealized appreciation was $48,700,576, representing approximately 76% of the Fund’s net assets. The Adviser no longer pursues a strategy of retaining unrealized long-term capital gain and avoiding the tax impact of realizing such gain. Subject to market conditions and Fund performance, the Adviser anticipates that, in managing the Fund’s investment portfolio in pursuit of the Fund’s investment objectives, a moderate portion of the Fund’s current built-in long-term capital gains will be realized gradually in each of the next several years. If these long-term capital gains are realized as anticipated, this will result in an increase in the Fund’s annual distributions of net capital gains and, accordingly, will generally result in taxable distributions to shareholders (other than certain shareholders that are exempt from tax on such income). Under normal market conditions, the Adviser currently expects, but does not guarantee, that no more than 15% of the total amount of the Fund’s current built-in long-term capital gains will be realized in any one year. This expectation may change over the course of the year for a variety of reasons including, but not limited to, individual holdings, market events and changes in tax laws. See “Dividends, Distributions and Taxes.” These risks are discussed in more detail later in this Prospectus and in the Statement of Additional Information (the “SAI”).
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	The Fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. To the extent the Fund invests its assets in fewer issuers, the Fund will be more susceptible to negative events affecting those issuers.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The accompanying bar chart and table provide some indication of the risks of investing in the Fund. The bar chart shows changes in the Fund’s performance from year to year. The table shows the average annual total returns of the Fund for 1, 5, and 10 years and also compares the Fund’s performance with the average annual total returns of an index or other benchmark. Updated performance information is available by calling the Fund at 877-363-6333. The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The accompanying bar chart and table provide some indication of the risks of investing in the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	877-363-6333
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Calendar Year Returns (before taxes)
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	The Fund’s calendar year-to-date return as of September 30, 2021 was 19.31%. During the period shown in the bar chart, the best performance for a quarter was 16.98% (for the quarter ended June 30, 2020) and the worst performance was (24.44)% (for the quarter ended March 31, 2020).
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2021
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	19.31%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	best performance
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2020
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	16.98%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	worst performance
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Mar. 31, 2020
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(24.44%)
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns (for Periods Ended December 31, 2020)
Index No Deduction for Fees, Expenses, Taxes [Text]	rr_IndexNoDeductionForFeesExpensesTaxes	reflects no deductions for fees, expenses, or taxes
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, and the after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).
Performance Table Explanation after Tax Higher	rr_PerformanceTableExplanationAfterTaxHigher	In certain cases, the figure representing “Return after taxes on distributions and sale of Fund shares” may be higher than the other figures for the same period. A higher after tax return results when a capital loss occurs upon redemption and provides an annual tax deduction that benefits the investor.
Performance Table Closing [Text Block]	rr_PerformanceTableClosingTextBlock	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, and the after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”). In certain cases, the figure representing “Return after taxes on distributions and sale of Fund shares” may be higher than the other figures for the same period. A higher after tax return results when a capital loss occurs upon redemption and provides an annual tax deduction that benefits the investor.
BARRETT OPPORTUNITY FUND, INC. \| S&P 500® Index (reflects no deduction for fees, expenses or taxes)
Prospectus [Line Items]	rr_ProspectusLineItems
Label	rr_AverageAnnualReturnLabel	S&P 500® Index(reflects no deductions for fees, expenses, or taxes)
1 Year	rr_AverageAnnualReturnYear01	18.40%
5 Years	rr_AverageAnnualReturnYear05	15.22%
10 Years	rr_AverageAnnualReturnYear10	13.88%
BARRETT OPPORTUNITY FUND, INC. \| Class O
Prospectus [Line Items]	rr_ProspectusLineItems
Trading Symbol	dei_TradingSymbol	SAOPX
Shareholder Fee, Other	rr_ShareholderFeeOther	none
Management Fees (as a percentage of Assets)	rr_ManagementFeesOverAssets	0.70%
Distribution and Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses (as a percentage of Assets):	rr_OtherExpensesOverAssets	0.48%
Expenses (as a percentage of Assets)	rr_ExpensesOverAssets	1.18%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 120
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	375
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	649
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 1,432
Annual Return 2011	rr_AnnualReturn2011	(5.91%)
Annual Return 2012	rr_AnnualReturn2012	12.77%
Annual Return 2013	rr_AnnualReturn2013	24.83%
Annual Return 2014	rr_AnnualReturn2014	5.99%
Annual Return 2015	rr_AnnualReturn2015	(2.94%)
Annual Return 2016	rr_AnnualReturn2016	17.46%
Annual Return 2017	rr_AnnualReturn2017	20.58%
Annual Return 2018	rr_AnnualReturn2018	(9.33%)
Annual Return 2019	rr_AnnualReturn2019	19.90%
Annual Return 2020	rr_AnnualReturn2020	1.23%
Label	rr_AverageAnnualReturnLabel	Return before taxes
1 Year	rr_AverageAnnualReturnYear01	1.23%
5 Years	rr_AverageAnnualReturnYear05	9.28%
10 Years	rr_AverageAnnualReturnYear10	7.82%
BARRETT OPPORTUNITY FUND, INC. \| Class O \| After Taxes on Distributions
Prospectus [Line Items]	rr_ProspectusLineItems
Label	rr_AverageAnnualReturnLabel	Return after taxes on distributions
1 Year	rr_AverageAnnualReturnYear01	(0.19%)
5 Years	rr_AverageAnnualReturnYear05	6.40%
10 Years	rr_AverageAnnualReturnYear10	5.33%
BARRETT OPPORTUNITY FUND, INC. \| Class O \| After Taxes on Distributions and Sales
Prospectus [Line Items]	rr_ProspectusLineItems
Label	rr_AverageAnnualReturnLabel	Return after taxes on distributions and sale of Fund shares
1 Year	rr_AverageAnnualReturnYear01	1.74%
5 Years	rr_AverageAnnualReturnYear05	7.01%
10 Years	rr_AverageAnnualReturnYear10	5.94%

Label

Element

Value

Prospectus [Line Items]

rr_ProspectusLineItems

Risk/Return [Heading]

rr_RiskReturnHeading

Barrett Opportunity Fund, Inc.

Objective [Heading]

rr_ObjectiveHeading

Investment Objectives

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

Barrett Opportunity Fund, Inc. (the “Fund”) seeks to achieve above average long-term capital appreciation.

Objective, Secondary [Text Block]

rr_ObjectiveSecondaryTextBlock

Current income is a secondary objective.

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 hold and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the table and example below.

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)

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 may generate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs and potentially higher taxes, 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 3% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

3.00%

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes:

•You invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods

•Your investment has a 5% return each year and the Fund’s operating expenses remain the same

You would incur these hypothetical expenses whether or not you redeem your investment at the end of the given period.

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

The Fund’s investment objectives are not fundamental and may be changed by the Board of Directors upon 60 days’ written notice to holders of the Fund’s common shares.

The Fund invests primarily in common stocks and securities convertible into or exchangeable for common stock such as convertible preferred stock or convertible debt securities.

The Fund may also invest without limit in fixed-income securities (including up to 5% of its net assets in fixed-income securities that are high yield, lower quality securities rated by a rating organization below its top four long-term rating categories (i.e., below investment grade securities, commonly referred to as “junk bonds”)) or unrated securities determined by the Adviser (as defined below) to be of equivalent quality.

The Fund is non-diversified.

Risk [Heading]

rr_RiskHeading

Principal Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

Risk is inherent in all investing. There is no assurance that the Fund will meet its investment objectives. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of certain risks of investing in the Fund.

Stock market and equity securities risk. The securities markets are volatile and the market prices of the Fund’s securities may decline generally. Securities fluctuate in price based on changes in a company’s financial condition and overall market and economic conditions, which are difficult to predict accurately. If the market prices of the securities owned by the Fund fall, the value of your investment in the Fund will decline.

Issuer risk. The value of a security can go up or down more than the market as a whole and can perform differently from the value of the market as a whole, often due to events such as (but not limited to) disappointing earnings reports by the issuer, unsuccessful products or services, loss of major customers, major litigation against the issuer or changes in government regulations affecting the issuer or the competitive environment. The Fund may experience a substantial or complete loss on an individual security. Historically, the prices of securities of small- and medium-capitalization companies have generally gone up or down more than those of large capitalization companies because, among other things, small- and medium-capitalization companies may be more sensitive to changing economic conditions and tend to be less established than large capitalization companies, although even large capitalization companies may fall out of favor with investors.

Foreign investments risk. The Fund’s investments in securities of foreign issuers involve greater risk than investments in securities of U.S. issuers. Foreign countries in which the Fund may invest may have markets that are less liquid and more volatile than U.S. markets and may suffer from political or economic instability. In some foreign countries, less information is available about issuers and markets because of less rigorous accounting, legal and regulatory standards than in the United States. Currency fluctuations could erase investment gains or add to investment losses. Income earned on foreign securities may be subject to foreign withholding taxes.

Portfolio selection risk. The value of your investment may decrease if the portfolio managers’ judgment about the attractiveness, value or market trends affecting a particular security, industry or sector or about market movements is incorrect.

Value investing risk. The value approach to investing involves the risk that stocks may remain undervalued. Value stocks may underperform the overall equity market. Although the Fund will not concentrate its investments in any one industry or industry group, it may, like many value funds, weight its investments toward certain industries, thus increasing its exposure to factors adversely affecting issuers within those industries. Accordingly, the Fund’s performance may be disproportionately affected by the poor performance of a certain industry.

Recent market events risk. U.S. and international markets have experienced significant periods of volatility in recent years due to a number of economic, political and global macro factors including the impact of the coronavirus (COVID-19) as a global pandemic which has resulted in public health issues, growth concerns in the U.S. and overseas, layoffs, rising unemployment and reduced consumer spending. The effects of COVID-19 may lead to a substantial economic downturn or recession in the U.S. and global economies, the recovery from which is uncertain and may last for an extended period of time.

Non-diversification risk. The Fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. To the extent the Fund invests its assets in fewer issuers, the Fund will be more susceptible to negative events affecting those issuers.

Cybersecurity risk. With the increased use of technologies such as the Internet to conduct business, the Fund is susceptible to operational, information security, and related risks. Cyber incidents affecting the

Fund or its service providers may cause disruptions and impact business operations, potentially resulting in financial losses, interference with the Fund’s ability to calculate its NAV, impediments to trading, the inability of shareholders to transact business, violations of applicable privacy and other laws, regulatory fines, penalties, reputational damage, reimbursement or other compensation costs, or additional compliance costs.

Risk of increase in expenses. Your actual costs of investing in the Fund may be higher than the expenses shown in “Annual Fund Operating Expenses” under “Fees and Expenses of the Fund” for a variety of reasons. For example, expense ratios may be higher than those shown if average net assets decrease. Net assets are more likely to decrease and fund expense ratios are more likely to increase when markets are volatile. Additionally, a higher portfolio turnover may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account.

Net unrealized appreciation. The Fund currently has a substantial amount of net unrealized appreciation. At November 30, 2021, the amount of the net unrealized appreciation was $48,700,576, representing approximately 76% of the Fund’s net assets. The Adviser no longer pursues a strategy of retaining unrealized long-term capital gain and avoiding the tax impact of realizing such gain. Subject to market conditions and Fund performance, the Adviser anticipates that, in managing the Fund’s investment portfolio in pursuit of the Fund’s investment objectives, a moderate portion of the Fund’s current built-in long-term capital gains will be realized gradually in each of the next several years. If these long-term capital gains are realized as anticipated, this will result in an increase in the Fund’s annual distributions of net capital gains and, accordingly, will generally result in taxable distributions to shareholders (other than certain shareholders that are exempt from tax on such income). Under normal market conditions, the Adviser currently expects, but does not guarantee, that no more than 15% of the total amount of the Fund’s current built-in long-term capital gains will be realized in any one year. This expectation may change over the course of the year for a variety of reasons including, but not limited to, individual holdings, market events and changes in tax laws. See “Dividends, Distributions and Taxes.”

These risks are discussed in more detail later in this Prospectus and in the Statement of Additional Information (the “SAI”).

Risk Lose Money [Text]

rr_RiskLoseMoney

You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.

Risk Nondiversified Status [Text]

rr_RiskNondiversifiedStatus

The Fund is classified as “non-diversified,” which means it may invest a larger percentage of its assets in a smaller number of issuers than a diversified fund. To the extent the Fund invests its assets in fewer issuers, the Fund will be more susceptible to negative events affecting those issuers.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

Performance

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The accompanying bar chart and table provide some indication of the risks of investing in the Fund. The bar chart shows changes in the Fund’s performance from year to year. The table shows the average annual total returns of the Fund for 1, 5, and 10 years and also compares the Fund’s performance with the average annual total returns of an index or other benchmark. Updated performance information is available by calling the Fund at 877-363-6333.

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The accompanying bar chart and table provide some indication of the risks of investing in the Fund.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

877-363-6333

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund’s past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Calendar Year Returns (before taxes)

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

The Fund’s calendar year-to-date return as of September 30, 2021 was 19.31%. During the period shown in the bar chart, the best performance for a quarter was 16.98% (for the quarter ended June 30, 2020) and the worst performance was (24.44)% (for the quarter ended March 31, 2020).

Year to Date Return, Label

rr_YearToDateReturnLabel

year-to-date return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2021

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

19.31%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

best performance

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2020

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

16.98%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

worst performance

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2020

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(24.44%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for Periods Ended December 31, 2020)

Index No Deduction for Fees, Expenses, Taxes [Text]

rr_IndexNoDeductionForFeesExpensesTaxes

reflects no deductions for fees, expenses, or taxes

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, and the after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).

Performance Table Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

In certain cases, the figure representing “Return after taxes on distributions and sale of Fund shares” may be higher than the other figures for the same period. A higher after tax return results when a capital loss occurs upon redemption and provides an annual tax deduction that benefits the investor.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

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, and the after-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).

BARRETT OPPORTUNITY FUND, INC. | S&P 500® Index (reflects no deduction for fees, expenses or taxes)

Prospectus [Line Items]

rr_ProspectusLineItems

Label

rr_AverageAnnualReturnLabel

S&P 500® Index(reflects no deductions for fees, expenses, or taxes)

1 Year

rr_AverageAnnualReturnYear01

18.40%

5 Years

rr_AverageAnnualReturnYear05

15.22%