The amount of unsecured line of credit terminated under debt arrangement.

No definition available.

Name:	gcbc_UnsecuredLineOfCreditFacilityTerminated
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480008/505-10-S99-1

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the declared dividend will be paid, in YYYY-MM-DD format.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-3

+ Details

Name:	us-gaap_DividendPayableDateToBePaidDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_DividendsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the dividend to be paid was declared, in YYYY-MM-DD format.

+ References

+ Details

Name:	us-gaap_DividendsPayableDateDeclaredDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the holder must own the stock to be entitled to the dividend, in YYYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_DividendsPayableDateOfRecordDayMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DividendsAxis=gcbc_DividendDeclaredCurrentFiscalYearAnnualMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=gcbc_AtlanticCommunityBankersBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DividendsAxis=gcbc_DividendDeclaredIn2024Q2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Subsequent events (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 17, 2024	Oct. 18, 2023	Jan. 18, 2024
2024 Annual Dividend [Member]
Dividends [Abstract]
Common stock, dividends declared (in dollars per share)		$ 0.32
Subsequent Event [Member] \| Atlantic Community Bankers Bank [Member]
Dividends [Abstract]
Unsecured line of credit, terminated amount			$ 7.5
Subsequent Event [Member] \| 2024 Annual Dividend [Member]
Dividends [Abstract]
Common stock, dividends declared (in dollars per share)	$ 0.32
Subsequent Event [Member] \| 2024 Q2 Dividend [Member]
Dividends [Abstract]
Dividends payable, date declared	Jan. 17, 2024
Common stock, dividends declared (in dollars per share)	$ 0.08
Dividends payable, date of record	Feb. 15, 2024
Dividends payable, date to be paid	Feb. 28, 2024