Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	GREEN CENTURY FUNDS
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018
Green Century MSCI International Index Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	GREEN CENTURY MSCI INTERNATIONAL INDEX FUND SUMMARY SECTION
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The Green Century MSCI International Index Fund seeks to achieve long-term total return which matches the performance of an index comprised of the stocks of foreign companies selected based on environmental, social and governance criteria.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. If you invest in shares of the Fund through an investment professional or financial intermediary, that investment professional or financial intermediary may charge you a commission in an amount determined and separately disclosed to you by that investment professional or financial intermediary.
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 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. For the period September 30, 2016 (commencement of operations) to July 31, 2017, the Fund’s portfolio turnover rate was 13% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	13.00%
Expense Example [Heading]	rr_ExpenseExampleHeading	Example
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This example is intended to help you compare the costs of investing in the Fund with the cost of investing in other mutual funds. This example assumes that: (1) 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; (2) your investment has a 5% return each year; and (3) the Fund’s operating expenses remain the same. 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 invests substantially all of its assets in the common stocks which make up the MSCI World ex USA SRI ex Fossil Fuels Index (the World ex USA SRI ex Fossil Fuels Index or the Index), a custom index calculated by MSCI, Inc. The World ex USA SRI ex Fossil Fuels Index is comprised of the common stocks of the companies in the MSCI World ex USA SRI Index (the World ex USA SRI Index), minus the stocks of the companies that explore for, extract, produce, manufacture or refine coal, oil or gas or produce or transmit electricity derived from fossil fuels or transmit natural gas or have carbon reserves included in the World ex USA SRI (Socially Responsible Investment) Index. The World ex USA SRI Index includes large and mid-cap stocks from approximately 22 developed markets countries (excluding the U.S.). The World ex USA SRI Index is a capitalization weighted index that provides exposure to companies with what MSCI calculates to have outstanding Environmental, Social and Governance (ESG) ratings and excludes companies whose products have negative social or environmental impacts. Green Century believes that those companies which pursue the least environmentally sound practices are at the greatest risk of brand and reputational damage and long term risk of negative economic consequences, while those which strive to be more environmentally responsible may benefit financially as a result. Green Century also believes that the use of ESG ratings creates an incentive for companies to become more sustainable. Companies involved in industries that may impose substantial risks and/or costs on society are evaluated based on their level of involvement, their social and environmental impact and their governance practices. The International Index Fund does not invest in, and the World ex USA SRI ex Fossil Fuels Index excludes, companies that explore for, extract, produce, manufacture or refine coal, oil or gas or produce or transmit electricity derived from fossil fuels or transmit natural gas or have carbon reserves. Companies determined to have significant business involvement in the following will not be included in the World ex USA SRI ex Fossil Fuels Index: genetically modified organisms (GMOs), firearms, military weapons, nuclear power, alcohol, tobacco, adult entertainment and gambling. The Fund buys and sells stocks so that the composition of its securities holdings will correspond, to the extent reasonably practicable, to the composition of securities in the World ex USA SRI ex Fossil Fuels Index. The weightings of the stocks in the World ex USA SRI ex Fossil Fuels Index are based on float-adjusted market capitalizations, which means the largest companies comprise a higher percentage of the World ex USA SRI ex Fossil Fuels Index and the Index is more heavily weighted in large than in smaller companies. As of January 31, 2018, the World ex USA SRI ex Fossil Fuels Index included companies with market capitalizations between approximately $2.6 billion and $173.4 billion. To the extent practicable, the Fund will seek a correlation between the weightings of securities held by the Fund and the weightings of the securities of the World ex USA SRI ex Fossil Fuels Index of 0.95 or better. A figure of 1.00 would indicate a perfect correlation. The Fund’s ability to duplicate the performance of the World ex USA SRI ex Fossil Fuels Index will depend to some extent on the size and timing of cash flows into and out of the Fund as well as the Fund’s expenses. The Fund may use an optimization strategy to achieve its investment objective, which means that it may not always hold the same securities in the same proportions as the World ex USA SRI ex Fossil Fuels Index. Especially when the Fund is small, the optimization strategy may result in the Fund not holding a significant number of securities included in the Index. Under normal circumstances and as a matter of operating policy, the Fund will invest at least 80% of its assets in the component securities of the World ex USA SRI ex Fossil Fuels Index and may invest in American Depository Receipts, Global Depository Receipts and Euro Depository Receipts representing the component securities of the Index.
Risk [Heading]	rr_RiskHeading	Principal Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	You may lose money by investing in the Fund. As with any mutual fund, there can be no guarantee that the Fund will achieve its objective. The following is a summary description of certain risks of investing in the Fund: Market Risk.    The value of securities held by the Fund may fall, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic or political conditions, inflation, changes in interest or currency rates, lack of liquidity in the bond markets or adverse investor sentiment. Changes in market conditions will not typically have the same impact on all types of securities. The values of securities may fall due to factors affecting a particular issuer, industry or the securities market as a whole. Financial markets throughout the world have experienced increased volatility, depressed valuations, decreased liquidity and heightened uncertainty. Governmental and non-governmental issuers have defaulted on, or been forced to restructure, their debts. These conditions may continue, recur, worsen or spread. Events that have contributed to these market conditions include, but are not limited to major cybersecurity events; geopolitical events (including wars and terror attacks); measures to address budget deficits; downgrading of sovereign debt; declines in oil and commodity prices; dramatic changes in currency exchange rates; and public sentiment. The U.S. government and the Federal Reserve, as well as certain foreign governments and central banks, have taken steps to support financial markets, including by keeping interest rates at historically low levels. This and other government intervention may not work as intended, particularly if the efforts are perceived by investors as being unlikely to achieve the desired results. The Federal Reserve has reduced its market support activities and recently has begun raising interest rates. Certain foreign governments and central banks are implementing or discussing so-called negative interest rates (e.g., charging depositors who keep their cash at a bank) to spur economic growth. Further Federal Reserve or other U.S. or non-U.S. governmental or central bank actions, including interest rate increases or contrary actions by different governments could negatively affect financial markets generally, increase market volatility and reduce the value and liquidity of securities in which the Fund invests. Policy and legislative changes in the United States and in other countries are affecting many aspects of financial regulation, and may in some instances contribute to decreased liquidity and increased volatility in the financial markets. The impact of these changes on the markets, and the practical implications for market participants, may not be fully known for some time. Economies and financial markets throughout the world are increasingly interconnected. Economic, financial or political events, trading and tariff arrangements, terrorism, natural disasters and other circumstances in one country or region could have profound impacts on global economies or markets. As a result, whether or not the Fund invests in securities of issuers located in or with significant exposure to the countries directly affected, the value and liquidity of the Fund’s investments may be negatively affected. Equity Securities Risk.    The Fund is heavily invested in stocks. Like all funds invested in stocks, the Fund’s share price will fluctuate daily depending on the performance of the companies that comprise the Fund’s investments, the general market and the economy overall. After you invest, the value of your shares may be less than what you paid for them. Large Cap Companies Risk.    Large-cap companies may be unable to respond quickly to new competitive challenges such as changes in technology, and also may not be able to attain the high growth rate of successful smaller companies, especially during extended periods of economic expansion. The large-cap companies in which the Fund invests may perform worse than the stock market as a whole. Mid-Cap Companies Risk.    The Fund may be invested in mid-cap companies which involve greater risk than investing in the stocks of larger, more established companies. Mid-cap companies may lack the management experience, financial resources and product diversification of large companies and the frequency and volume of their trading may be less than that of larger companies. Therefore, securities of mid-cap companies may be subject to wider and more erratic price fluctuations. Risks of Non-U.S. Investments.    Investing in non-U.S. issuers or in securities of U.S. issuers with significant exposure to foreign markets may involve unique risks compared to investing in securities of U.S. issuers. These risks are more pronounced to the extent that the Fund invests significantly in one region or country. These risks may include different financial reporting practices and regulatory standards, less liquid trading markets, currency risks, changes in economic, political, regulatory and social conditions, sustained economic downturns, tax burdens, natural disasters, terrorism and investment and repatriation restrictions. Liquidity Risk.    Liquidity risk exists when particular investments are difficult to purchase or sell. When the Fund holds these types of investments, the Fund’s portfolio may be more difficult to value, especially during periods of market turmoil. Markets may become illiquid when there are few, if any, interested buyers or sellers or when dealers are unwilling or unable to make a market for certain securities. As a general matter, dealers recently have been less willing to make markets for fixed income securities. When a Fund holds illiquid investments, the Fund’s portfolio may be harder to value, especially in changing markets. Investments by the Fund in derivatives, below investment grade securities, foreign securities, and corporate loans tend to involve greater liquidity risk. If a Fund is forced to sell or unwind these investments to meet redemptions or for other cash needs, the Fund may suffer a loss. Additionally, the market for certain investments may become illiquid under adverse market or economic conditions independent of any specific adverse changes in the conditions of a particular issuer. In such cases, the Fund, due to limitations on investments in illiquid securities and the difficulty in purchasing and selling such securities, may be unable to achieve its desired level of exposure to certain sectors. Further, certain securities, once sold, may not settle for an extended period. A Fund will not receive its sales proceeds until that time, which may constrain the Fund’s ability to meet its obligations (including obligations to redeeming shareholders). Index Fund Risk.    The Fund will invest in the stocks composing the World ex USA SRI ex Fossil Fuels Index regardless of how the Index is performing. It will not shift concentration from one industry to another, or from stocks to bonds or cash, in order to defend against a falling stock market. The Index may, at times, become focused in stocks of a particular sector, category or group of companies. Because the Fund seeks to track the World ex USA SRI ex Fossil Fuels Index, the Fund may underperform the overall stock market. Non-Correlation Risk.    The performance of the Fund and of the World ex USA SRI ex Fossil Fuels Index may vary for a variety of reasons. For example, the Fund incurs operating expenses and portfolio transaction costs not incurred by the Index. In addition, the Fund may not be fully invested in the component securities of the Index. Use of sampling techniques particularly when the Fund is small may affect the Fund’s ability to achieve close correlation with the Index. Valuation Risk.    The sales price the Fund could receive for any particular portfolio investment may differ from the Fund’s valuation of the investment, particularly for securities that trade in thin or volatile markets or that are valued using a fair value methodology. Investors who purchase or redeem Fund shares on days when the Fund is holding fair-valued securities may receive fewer or more shares or lower or higher redemption proceeds than they would have received if the Fund had not fair-valued securities or had used a different valuation methodology. The Fund’s ability to value its investments may be impacted by technological issues and/or errors by pricing services or other third party service providers. Environmentally and Socially Responsible Investing Risk.    The Fund’s environmental, social and governance criteria limit the available investments compared with funds with no such criteria. Under certain economic conditions, this could cause the Fund’s investment performance to be worse or better than similar funds with no such criteria. Redemption Risk.    The Fund may experience periods of heavy redemptions that could cause the Fund to liquidate its assets at inopportune times or at a loss or depressed value, particularly during periods of declining or illiquid markets. Redemption risk is greater to the extent that the Fund has investors with large shareholdings, short investment horizons, or unpredictable cash flow needs. In addition, redemption risk is heightened during periods of overall market turmoil. The redemption by one or more large shareholders of their holdings in the Fund could hurt performance and/or cause the remaining shareholders in the Fund to lose money. Further, if one decision maker has control of Fund shares owned by separate Fund shareholders, including clients or affiliates of the Fund’s adviser, redemptions by these shareholders may further increase the Fund’s redemption risk. If the Fund is forced to liquidate its assets under unfavorable conditions or at inopportune times, the value of your investment could decline. These and other risks are discussed in more detail in “Additional Information About the Funds’ Investment Objectives, Strategies and Risks” in this Prospectus and in the Statement of Additional Information.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose money by investing in the Fund.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The bar chart and the average annual total return table below provide some indication of the risks of investing in the Fund by showing how the Fund has performed in the past. The bar chart shows changes in the performance of the Fund’s Individual Investor Class from year to year. The table shows how the average annual total return for each class of the Fund for the one-year and since-inception periods compare with the returns of a broad measure of market performance. The Fund’s past performance (before and after taxes) does not necessarily indicate how well it will perform in the future. Updated performance information is available on the Fund’s website, www.greencentury.com/ the-green-century-funds / performance, or by calling 1-800-93-GREEN.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The bar chart and the average annual total return table below provide some indication of the risks of investing in the Fund by showing how the Fund has performed in the past. The bar chart shows changes in the performance of the Fund’s Individual Investor Class from year to year. The table shows how the average annual total return for each class of the Fund for the one-year and since-inception periods compare with the returns of a broad measure of market performance.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-800-93-GREEN
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	www.greencentury.com/ the-green-century-funds / performance
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	The Fund’s past performance (before and after taxes) does not necessarily indicate how well it will perform in the future.
Bar Chart [Heading]	rr_BarChartHeading	Annual Total Return for the Year Ended December 31, 2017
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the period shown, the best quarterly performance of the Fund's Individual Investor Class was 7.95%, for the quarter ended 3/31/17. The worst quarterly performance of the Fund's Individual Investor Class was 3.35%, for the quarter ended 12/31/17.
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Total Returns for the Periods Ended December 31, 2017
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	The 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	The after-tax returns are shown for the Individual Investor Class of the Fund and the after-tax returns for the Institutional Class of the Fund will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	The after-tax returns are shown for the Individual Investor Class of the Fund and the after-tax returns for the Institutional Class of the Fund will vary. The 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.
Green Century MSCI International Index Fund | Individual Investor Class
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load)	rr_MaximumDeferredSalesChargeOverOther	none
Redemption Fee (as a percentage of an amount redeemed within 60 days of purchase)	rr_RedemptionFeeOverRedemption	2.00%
Wire Redemption Fee (if such services are requested)	gcf_WireRedemptionFeeIfSuchServicesAreRequested	$ 10
Overnight Delivery Fee (if such services are requested)	gcf_OvernightDeliveryFeeIfSuchServicesAreRequested	$ 10
Management Fees	rr_ManagementFeesOverAssets	0.28%
Distribution (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Administrative Fees	rr_Component1OtherExpensesOverAssets	1.00%
Other Fees	rr_Component2OtherExpensesOverAssets	none
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.28%
1 Year	rr_ExpenseExampleYear01	$ 130
3 Years	rr_ExpenseExampleYear03	406
5 Years	rr_ExpenseExampleYear05	702
10 Years	rr_ExpenseExampleYear10	$ 1,545
2017	rr_AnnualReturn2017	23.85%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	best quarterly performance
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2017
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	7.95%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	worst quarterly performance
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2017
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	3.35%
1 Year	rr_AverageAnnualReturnYear01	23.85%
Since Inception	rr_AverageAnnualReturnSinceInception	15.75%
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 30, 2016
Green Century MSCI International Index Fund | Institutional Class
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load)	rr_MaximumDeferredSalesChargeOverOther	none
Redemption Fee (as a percentage of an amount redeemed within 60 days of purchase)	rr_RedemptionFeeOverRedemption	2.00%
Wire Redemption Fee (if such services are requested)	gcf_WireRedemptionFeeIfSuchServicesAreRequested	$ 15
Overnight Delivery Fee (if such services are requested)	gcf_OvernightDeliveryFeeIfSuchServicesAreRequested	$ 15
Management Fees	rr_ManagementFeesOverAssets	0.28%
Distribution (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Administrative Fees	rr_Component1OtherExpensesOverAssets	0.70%
Other Fees	rr_Component2OtherExpensesOverAssets	none
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.98%
1 Year	rr_ExpenseExampleYear01	$ 100
3 Years	rr_ExpenseExampleYear03	312
5 Years	rr_ExpenseExampleYear05	542
10 Years	rr_ExpenseExampleYear10	$ 1,201
1 Year	rr_AverageAnnualReturnYear01	24.13%
Since Inception	rr_AverageAnnualReturnSinceInception	15.99%
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 30, 2016
Green Century MSCI International Index Fund | Return after taxes on distributions | Individual Investor Class
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	23.38%
Since Inception	rr_AverageAnnualReturnSinceInception	15.36%
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 30, 2016
Green Century MSCI International Index Fund | Return after taxes on distributions and sale of Fund shares | Individual Investor Class
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	13.77%
Since Inception	rr_AverageAnnualReturnSinceInception	11.19%
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 30, 2016
Green Century MSCI International Index Fund | MSCI World Ex USA Index (reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	24.21%
Since Inception	rr_AverageAnnualReturnSinceInception	18.56%
Inception Date	rr_AverageAnnualReturnInceptionDate	Sep. 30, 2016