Basis of Presentation (Policies)	6 Months Ended
Dec. 31, 2020
Basis of Presentation [Abstract]
Basis of Presentation	Within the accompanying unaudited consolidated statement of financial condition, and related notes to the consolidated financial statements, June 30, 2020 data was derived from the audited consolidated financial statements of Greene County Bancorp, Inc. (the “Company”) and its wholly owned subsidiaries, The Bank of Greene County (the “Bank”) and Greene Risk Management, Inc., and the Bank’s wholly owned subsidiaries, Greene County Commercial Bank and Greene Property Holdings, Ltd. The consolidated financial statements at and for the three and six months ended December 31, 2020 and 2019 are unaudited. The financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and with the instructions to Form 10-Q and Article 8 of Regulation S-X. Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. To the extent that information and notes required by GAAP for complete financial statements are contained in or are consistent with the audited financial statements incorporated by reference to Greene County Bancorp, Inc.’s Annual Report on Form 10-K for the year ended June 30, 2020, such information and notes have not been duplicated herein. In the opinion of management, all adjustments (consisting of only normal recurring items) necessary for a fair presentation of the financial position and results of operations and cash flows at and for the periods presented have been included. The Company had no reclassifications from amounts in the prior year’s consolidated financial statements to conform to the current year’s presentation. All material inter-company accounts and transactions have been eliminated in the consolidation. The results of operations and other data for the three and six months ended December 31, 2020 are not necessarily indicative of results that may be expected for the entire fiscal year ending June 30, 2021. These consolidated financial statements consider events that occurred through the date the consolidated financial statements were issued. In December 2019, an outbreak of a novel strain of coronavirus (“COVID-19”) originated in Wuhan, China and has since spread to other countries, including the U.S. On March 11, 2020, the World Health Organization characterized COVID-19 as a pandemic. In addition, multiple jurisdictions in the U.S. have declared states of emergency. It is anticipated that these impacts will continue for some time. Potential impacts to the Company include disruptions or restrictions on our employees’ ability to work, lack of demand for new loans or the borrower’s ability to pay the required monthly payments. Changes to the operating environment may also be impacted. Operations include loan applications, processing or other areas requiring contact with the borrower. These changes may increase operating costs. Further impacts may include increased repurchase risk or loan defaults. The future effects of these issues are unknown. CRITICAL ACCOUNTING POLICIES Greene County Bancorp, Inc.’s critical accounting policies relate to the allowance for loan losses and the evaluation of securities for other-than-temporary impairment. The allowance for loan losses is based on management’s estimation of an amount that is intended to absorb losses in the existing portfolio. The allowance for loan losses is established through a provision for loan losses based on management’s evaluation of the risk inherent in the loan portfolio, the composition of the portfolio, specific impaired loans and current economic conditions. Such evaluation, which includes a review of all loans for which full collectability may not be reasonably assured, considers among other matters, the estimated net realizable value or the fair value of the underlying collateral, economic conditions, historical loan loss experience, management’s estimate of probable credit losses and other factors that warrant recognition in providing for the allowance of loan losses. However, this evaluation involves a high degree of complexity and requires management to make subjective judgments that often require assumptions or estimates about highly uncertain matters. This critical accounting policy and its application are periodically reviewed with the Audit Committee and the Board of Directors. There have been no significant changes in the application of this critical accounting policy during the three and six months ended December 31, 2020. Securities are evaluated for other-than-temporary impairment by performing periodic reviews of individual securities in the investment portfolio. Greene County Bancorp, Inc. makes an assessment to determine whether there have been any events or economic circumstances to indicate that a security, on which there is an unrealized loss, is impaired on an other-than-temporary basis. The Company considers many factors, including the severity and duration of the impairment; the intent and ability of the Company to hold the equity security for a period of time sufficient for a recovery in value; recent events specific to the issuer or industry; and for debt securities, the intent to sell the security, the likelihood to be required to sell the security before it recovers the entire amortized cost, external credit ratings and recent downgrades. The Company is required to record other-than-temporary impairment charges through earnings, if it has the intent to sell, or will more likely than not be required to sell an impaired debt security before a recovery of its amortized cost basis. In addition, the Company is required to record other-than-temporary impairment charges through earnings for the amount of credit losses, regardless of the intent or requirement to sell. Credit loss is measured as the difference between the present value of an impaired debt security’s cash flows and its amortized cost basis. Non-credit related write-downs to fair value must be recorded as decreases to accumulated other comprehensive income as long as the Company has no intent or requirement to sell an impaired security before a recovery of amortized cost basis.

Within the accompanying unaudited consolidated statement of financial condition, and related notes to the consolidated financial statements, June 30, 2020 data was derived from the audited consolidated financial statements of Greene County Bancorp, Inc. (the “Company”) and its wholly owned subsidiaries, The Bank of Greene County (the “Bank”) and Greene Risk Management, Inc., and the Bank’s wholly owned subsidiaries, Greene County Commercial Bank and Greene Property Holdings, Ltd. The consolidated financial statements at and for the three and six months ended December 31, 2020 and 2019 are unaudited.

The financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information and with the instructions to Form 10-Q and Article 8 of Regulation S-X. Accordingly, they do not include all of the information and notes required by GAAP for complete financial statements. To the extent that information and notes required by GAAP for complete financial statements are contained in or are consistent with the audited financial statements incorporated by reference to Greene County Bancorp, Inc.’s Annual Report on Form 10-K for the year ended June 30, 2020, such information and notes have not been duplicated herein. In the opinion of management, all adjustments (consisting of only normal recurring items) necessary for a fair presentation of the financial position and results of operations and cash flows at and for the periods presented have been included. The Company had no reclassifications from amounts in the prior year’s consolidated financial statements to conform to the current year’s presentation. All material inter-company accounts and transactions have been eliminated in the consolidation. The results of operations and other data for the three and six months ended December 31, 2020 are not necessarily indicative of results that may be expected for the entire fiscal year ending June 30, 2021. These consolidated financial statements consider events that occurred through the date the consolidated financial statements were issued.

In December 2019, an outbreak of a novel strain of coronavirus (“COVID-19”) originated in Wuhan, China and has since spread to other countries, including the U.S. On March 11, 2020, the World Health Organization characterized COVID-19 as a pandemic. In addition, multiple jurisdictions in the U.S. have declared states of emergency. It is anticipated that these impacts will continue for some time. Potential impacts to the Company include disruptions or restrictions on our employees’ ability to work, lack of demand for new loans or the borrower’s ability to pay the required monthly payments. Changes to the operating environment may also be impacted. Operations include loan applications, processing or other areas requiring contact with the borrower. These changes may increase operating costs. Further impacts may include increased repurchase risk or loan defaults. The future effects of these issues are unknown.

CRITICAL ACCOUNTING POLICIES

Greene County Bancorp, Inc.’s critical accounting policies relate to the allowance for loan losses and the evaluation of securities for other-than-temporary impairment. The allowance for loan losses is based on management’s estimation of an amount that is intended to absorb losses in the existing portfolio. The allowance for loan losses is established through a provision for loan losses based on management’s evaluation of the risk inherent in the loan portfolio, the composition of the portfolio, specific impaired loans and current economic conditions. Such evaluation, which includes a review of all loans for which full collectability may not be reasonably assured, considers among other matters, the estimated net realizable value or the fair value of the underlying collateral, economic conditions, historical loan loss experience, management’s estimate of probable credit losses and other factors that warrant recognition in providing for the allowance of loan losses. However, this evaluation involves a high degree of complexity and requires management to make subjective judgments that often require assumptions or estimates about highly uncertain matters. This critical accounting policy and its application are periodically reviewed with the Audit Committee and the Board of Directors. There have been no significant changes in the application of this critical accounting policy during the three and six months ended December 31, 2020.

Securities are evaluated for other-than-temporary impairment by performing periodic reviews of individual securities in the investment portfolio. Greene County Bancorp, Inc. makes an assessment to determine whether there have been any events or economic circumstances to indicate that a security, on which there is an unrealized loss, is impaired on an other-than-temporary basis. The Company considers many factors, including the severity and duration of the impairment; the intent and ability of the Company to hold the equity security for a period of time sufficient for a recovery in value; recent events specific to the issuer or industry; and for debt securities, the intent to sell the security, the likelihood to be required to sell the security before it recovers the entire amortized cost, external credit ratings and recent downgrades. The Company is required to record other-than-temporary impairment charges through earnings, if it has the intent to sell, or will more likely than not be required to sell an impaired debt security before a recovery of its amortized cost basis. In addition, the Company is required to record other-than-temporary impairment charges through earnings for the amount of credit losses, regardless of the intent or requirement to sell. Credit loss is measured as the difference between the present value of an impaired debt security’s cash flows and its amortized cost basis. Non-credit related write-downs to fair value must be recorded as decreases to accumulated other comprehensive income as long as the Company has no intent or requirement to sell an impaired security before a recovery of amortized cost basis.