Securities, Securities in Continuous Unrealized Loss Position (Details) $ in Thousands	12 Months Ended
Jun. 30, 2021 USD ($) Security	Jun. 30, 2020 USD ($) Security
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 87,974	$ 3,538
More than 12 months, fair value	2,469	0
Total, fair value	90,443	3,538
Less than 12 months, unrealized losses	1,343	17
More than 12 months, unrealized losses	61	0
Total, unrealized losses	$ 1,404	$ 17
Less than 12 months, number of securities \| Security	29	4
More than 12 months, number of securities \| Security	1	0
Total, number of securities \| Security	30	4
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 63,977	$ 11,605
More than 12 months, fair value	471	361
Total, fair value	64,448	11,966
Less than 12 months, unrealized losses	443	41
More than 12 months, unrealized losses	29	128
Total, unrealized losses	$ 472	$ 169
Less than 12 months, number of securities \| Security	84	19
More than 12 months, number of securities \| Security	1	1
Total, number of securities \| Security	85	20
Total Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 151,951	$ 15,143
More than 12 months, fair value	2,940	361
Total, fair value	154,891	15,504
Less than 12 months, unrealized losses	1,786	58
More than 12 months, unrealized losses	90	128
Total, unrealized losses	$ 1,876	$ 186
Less than 12 months, number of securities \| Security	113	23
More than 12 months, number of securities \| Security	2	1
Total, number of securities \| Security	115	24
Available for sale securities transferred at fair value to held to maturity	$ 0	$ 0
Proceeds from sale of available-for-sale securities	0	0
Gross realized gains (losses) on sale of available-for-sale securities	0	0
Other-than-temporary impairment losses	0	0
U.S. Government Sponsored Enterprises [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	6,787
More than 12 months, fair value	0
Total, fair value	6,787
Less than 12 months, unrealized losses	212
More than 12 months, unrealized losses	0
Total, unrealized losses	$ 212
Less than 12 months, number of securities \| Security	2
More than 12 months, number of securities \| Security	0
Total, number of securities \| Security	2
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Total, unrealized losses		0
U.S. Treasury Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 1,970
More than 12 months, fair value	0
Total, fair value	1,970
Less than 12 months, unrealized losses	1
More than 12 months, unrealized losses	0
Total, unrealized losses	$ 1
Less than 12 months, number of securities \| Security	1
More than 12 months, number of securities \| Security	0
Total, number of securities \| Security	1
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 1,991
More than 12 months, fair value	0
Total, fair value	1,991
Less than 12 months, unrealized losses	2
More than 12 months, unrealized losses	0
Total, unrealized losses	$ 2
Less than 12 months, number of securities \| Security	1
More than 12 months, number of securities \| Security	0
Total, number of securities \| Security	1
State and Political Subdivisions [Member]
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 42,751	3,336
More than 12 months, fair value	0	0
Total, fair value	42,751	3,336
Less than 12 months, unrealized losses	303	3
More than 12 months, unrealized losses	0	0
Total, unrealized losses	$ 303	$ 3
Less than 12 months, number of securities \| Security	76	12
More than 12 months, number of securities \| Security	0	0
Total, number of securities \| Security	76	12
Mortgage-backed Securities-Residential [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 19,071
More than 12 months, fair value	0
Total, fair value	19,071
Less than 12 months, unrealized losses	167
More than 12 months, unrealized losses	0
Total, unrealized losses	$ 167
Less than 12 months, number of securities \| Security	4
More than 12 months, number of securities \| Security	0
Total, number of securities \| Security	4
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 12,839	$ 3,604
More than 12 months, fair value	0	0
Total, fair value	12,839	3,604
Less than 12 months, unrealized losses	90	15
More than 12 months, unrealized losses	0	0
Total, unrealized losses	$ 90	$ 15
Less than 12 months, number of securities \| Security	2	2
More than 12 months, number of securities \| Security	0	0
Total, number of securities \| Security	2	2
Mortgage-backed Securities-Multi-Family [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 59,176	$ 1,051
More than 12 months, fair value	2,469	0
Total, fair value	61,645	1,051
Less than 12 months, unrealized losses	933	4
More than 12 months, unrealized losses	61	0
Total, unrealized losses	$ 994	$ 4
Less than 12 months, number of securities \| Security	21	1
More than 12 months, number of securities \| Security	1	0
Total, number of securities \| Security	22	1
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 3,890	$ 3,562
More than 12 months, fair value	0	0
Total, fair value	3,890	3,562
Less than 12 months, unrealized losses	12	21
More than 12 months, unrealized losses	0	0
Total, unrealized losses	$ 12	$ 21
Less than 12 months, number of securities \| Security	3	3
More than 12 months, number of securities \| Security	0	0
Total, number of securities \| Security	3	3
Corporate Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 970	$ 2,487
More than 12 months, fair value	0	0
Total, fair value	970	2,487
Less than 12 months, unrealized losses	30	13
More than 12 months, unrealized losses	0	0
Total, unrealized losses	$ 30	$ 13
Less than 12 months, number of securities \| Security	1	3
More than 12 months, number of securities \| Security	0	0
Total, number of securities \| Security	1	3
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 2,506	$ 1,103
More than 12 months, fair value	471	361
Total, fair value	2,977	1,464
Less than 12 months, unrealized losses	36	2
More than 12 months, unrealized losses	29	128
Total, unrealized losses	$ 65	$ 130
Less than 12 months, number of securities \| Security	2	2
More than 12 months, number of securities \| Security	1	1
Total, number of securities \| Security	3	3

No definition available.

Name:	gcbc_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionAbstract
Namespace Prefix:	gcbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the net carrying amount of the transferred security to another category of investments i.e. value transferred from available for sale to held to maturity.

+ References

No definition available.

+ Details

Name:	gcbc_AvailableForSaleSecuritiesTransfersToHeldToMaturity
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of investment positions in held-to-maturity investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment positions in held-to-maturity investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	gcbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment positions in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	gcbc_NumberOfSecuritiesUnrealizedLossPosition
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment positions in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	gcbc_NumberOfSecuritiesUnrealizedLossPositionLessThanTwelveMonths
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of investment positions that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

+ Details

Name:	gcbc_NumberOfSecuritiesUnrealizedLossPositionTwelveMonthsOrLonger
Namespace Prefix:	gcbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents the excess of amortized cost basis over fair value of securities in a continuous loss position for 12 months or greater.

+ References

No definition available.

+ Details

Name:	gcbc_SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	gcbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the excess of amortized cost basis over fair value of securities in a continuous loss position for less than 12 months.

+ References

No definition available.

+ Details

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

- Definition

This item represents the fair value of securities in a continuous loss position for less than twelve months.

+ References

No definition available.

+ Details

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

- Definition

This item represents the fair value of securities in a continuous loss position for a period greater than 12 months.

+ References

No definition available.

+ Details

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

- Definition

Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position.

+ References

No definition available.

+ Details

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

- Definition

This item represents the fair value of securities that have been in a continuous unrealized loss position.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922888-210455

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27357-111563

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=SL120269820-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for 12 months or longer.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=124260329&loc=SL6284422-111562

+ Details

Name:	us-gaap_OtherThanTemporaryImpairmentLossDebtSecuritiesAvailableForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3151-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=gcbc_MortgageBackedSecuritiesMultiFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: