XML 191 R56.htm IDEA: XBRL DOCUMENT
v3.25.2

Securities - Securities in Continuous Unrealized Loss Position (Details) $ in Thousands | Jun. 30, 2025 USD ($) Security | Jun. 30, 2024 USD ($) Security
--- | --- | ---
Securities available-for-sale: |  |
Less than 12 months, fair value | $ 241 | $ 26,487
Less than 12 months, unrealized losses | $ 1 | $ 223
Less than 12 months, number of securities (in Security) | Security | 1 | 3
More than 12 months, fair value | $ 133,287 | $ 139,017
More than 12 months, unrealized losses | $ 19,590 | $ 26,865
More than 12 months, number of securities | 73 | 84
Total, fair value | $ 133,528 | $ 165,504
Total, unrealized losses | $ 19,591 | $ 27,088
Total, number of securities | 74 | 87
Securities held-to-maturity: |  |
Less than 12 months, fair value | $ 36,752 | $ 32,215
Less than 12 months, unrealized losses | $ 535 | $ 474
Less than 12 months, number of securities (in Security) | Security | 161 | 294
More than 12 months, fair value | $ 413,533 | $ 476,309
More than 12 months, unrealized losses | $ 48,407 | $ 64,726
More than 12 months, number of securities | 1,604 | 2,148
Total, fair value | $ 450,285 | $ 508,524
Total, unrealized losses | $ 48,942 | $ 65,200
Total, number of securities | 1,765 | 2,442
Less than 12 months, fair value | $ 36,993 | $ 58,702
Less than 12 months, unrealized losses | $ 536 | $ 697
Less than 12 months, number of securities (in Security) | Security | 162 | 297
More than 12 months, fair value | $ 546,820 | $ 615,326
More than 12 months, unrealized losses | $ 67,997 | $ 91,591
More than 12 months, number of securities | 1,677 | 2,232
Total, fair value | $ 583,813 | $ 674,028
Total, unrealized losses | $ 68,533 | $ 92,288
Total, number of securities | 1,839 | 2,529
U.S. Treasury Securities [Member] |  |
Securities available-for-sale: |  |
Less than 12 months, fair value | $ 241 | $ 24,574
Less than 12 months, unrealized losses | $ 1 | $ 215
Less than 12 months, number of securities (in Security) | Security | 1 | 1
More than 12 months, fair value | $ 10,212 | $ 16,621
More than 12 months, unrealized losses | $ 361 | $ 1,614
More than 12 months, number of securities | 5 | 8
Total, fair value | $ 10,453 | $ 41,195
Total, unrealized losses | $ 362 | $ 1,829
Total, number of securities | 6 | 9
Securities held-to-maturity: |  |
Less than 12 months, fair value | $ 0 | $ 0
Less than 12 months, unrealized losses | $ 0 | $ 0
Less than 12 months, number of securities (in Security) | Security | 0 | 0
More than 12 months, fair value | $ 14,982 | $ 22,036
More than 12 months, unrealized losses | $ 868 | $ 1,749
More than 12 months, number of securities | 5 | 7
Total, fair value | $ 14,982 | $ 22,036
Total, unrealized losses | $ 868 | $ 1,749
Total, number of securities | 5 | 7
U.S. Government Sponsored Enterprises [Member] |  |
Securities available-for-sale: |  |
Less than 12 months, fair value | $ 0 | $ 0
Less than 12 months, unrealized losses | $ 0 | $ 0
Less than 12 months, number of securities (in Security) | Security | 0 | 0
More than 12 months, fair value | $ 11,644 | $ 10,974
More than 12 months, unrealized losses | $ 1,385 | $ 2,068
More than 12 months, number of securities | 5 | 5
Total, fair value | $ 11,644 | $ 10,974
Total, unrealized losses | $ 1,385 | $ 2,068
Total, number of securities | 5 | 5
State and Political Subdivisions [Member] |  |
Securities available-for-sale: |  |
Less than 12 months, fair value | $ 0 | $ 0
Less than 12 months, unrealized losses | $ 0 | $ 0
Less than 12 months, number of securities (in Security) | Security | 0 | 0
More than 12 months, fair value | $ 43 | $ 62
More than 12 months, unrealized losses | $ 0 | $ 1
More than 12 months, number of securities | 1 | 1
Total, fair value | $ 43 | $ 62
Total, unrealized losses | $ 0 | $ 1
Total, number of securities | 1 | 1
Securities held-to-maturity: |  |
Less than 12 months, fair value | $ 23,577 | $ 32,215
Less than 12 months, unrealized losses | $ 339 | $ 474
Less than 12 months, number of securities (in Security) | Security | 154 | 294
More than 12 months, fair value | $ 231,645 | $ 278,521
More than 12 months, unrealized losses | $ 31,689 | $ 39,761
More than 12 months, number of securities | 1,511 | 2,025
Total, fair value | $ 255,222 | $ 310,736
Total, unrealized losses | $ 32,028 | $ 40,235
Total, number of securities | 1,665 | 2,319
Mortgage-backed Securities-Residential [Member] |  |
Securities available-for-sale: |  |
Less than 12 months, fair value | $ 0 | $ 1,913
Less than 12 months, unrealized losses | 0 | $ 8
Less than 12 months, number of securities (in Security) | Security |  | 2
More than 12 months, fair value | 20,872 | $ 22,700
More than 12 months, unrealized losses | $ 3,007 | $ 3,886
More than 12 months, number of securities | 21 | 23
Total, fair value | $ 20,872 | $ 24,613
Total, unrealized losses | $ 3,007 | $ 3,894
Total, number of securities | 21 | 25
Securities held-to-maturity: |  |
Less than 12 months, fair value | $ 9,470 | $ 0
Less than 12 months, unrealized losses | $ 151 | $ 0
Less than 12 months, number of securities (in Security) | Security | 4 | 0
More than 12 months, fair value | $ 26,541 | $ 29,510
More than 12 months, unrealized losses | $ 2,176 | $ 3,314
More than 12 months, number of securities | 26 | 28
Total, fair value | $ 36,011 | $ 29,510
Total, unrealized losses | $ 2,327 | $ 3,314
Total, number of securities | 30 | 28
Mortgage-backed Securities-Multi-family [Member] |  |
Securities available-for-sale: |  |
Less than 12 months, fair value | $ 0 | $ 0
Less than 12 months, unrealized losses | $ 0 | $ 0
Less than 12 months, number of securities (in Security) | Security | 0 | 0
More than 12 months, fair value | $ 74,597 | $ 72,300
More than 12 months, unrealized losses | $ 14,277 | $ 17,961
More than 12 months, number of securities | 30 | 31
Total, fair value | $ 74,597 | $ 72,300
Total, unrealized losses | $ 14,277 | $ 17,961
Total, number of securities | 30 | 31
Securities held-to-maturity: |  |
Less than 12 months, fair value | $ 0 | $ 0
Less than 12 months, unrealized losses | $ 0 | $ 0
Less than 12 months, number of securities (in Security) | Security | 0 | 0
More than 12 months, fair value | $ 118,156 | $ 125,966
More than 12 months, unrealized losses | $ 11,963 | $ 17,397
More than 12 months, number of securities | 45 | 47
Total, fair value | $ 118,156 | $ 125,966
Total, unrealized losses | $ 11,963 | $ 17,397
Total, number of securities | 45 | 47
Corporate Debt Securities [Member] |  |
Securities available-for-sale: |  |
Less than 12 months, fair value | $ 0 | $ 0
Less than 12 months, unrealized losses | $ 0 | $ 0
Less than 12 months, number of securities (in Security) | Security | 0 | 0
More than 12 months, fair value | $ 15,919 | $ 16,360
More than 12 months, unrealized losses | $ 560 | $ 1,335
More than 12 months, number of securities | 11 | 16
Total, fair value | $ 15,919 | $ 16,360
Total, unrealized losses | $ 560 | $ 1,335
Total, number of securities | 11 | 16
Securities held-to-maturity: |  |
Less than 12 months, fair value | $ 3,705 | $ 0
Less than 12 months, unrealized losses | $ 45 | $ 0
Less than 12 months, number of securities (in Security) | Security | 3 | 0
More than 12 months, fair value | $ 22,209 | $ 20,276
More than 12 months, unrealized losses | $ 1,711 | $ 2,505
More than 12 months, number of securities | 17 | 41
Total, fair value | $ 25,914 | $ 20,276
Total, unrealized losses | $ 1,756 | $ 2,505
Total, number of securities | 20 | 41

X
- 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 No definition available. + Details
Name: | gcbc_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositions12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 No definition available. + Details
Name: | gcbc_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositions12MonthsOrLongerFairValue
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 No definition available. + Details
Name: | gcbc_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositionsLessThan12MonthsAccumulatedLoss
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 No definition available. + Details
Name: | gcbc_DebtSecuritiesHeldToMaturityContinuousUnrealizedLossPositionsLessThan12MonthsFairValue
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position. + References No definition available. + Details
Name: | gcbc_DebtSecuritiesHeldToMaturityUnrealizedLossPositionsFairValue
Namespace Prefix: | gcbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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

X
- 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

X
- Definition Number of investment positions in unrealized loss position for held-to-maturity security where cost exceeds fair value and its impairment is not other-than-temporary. + References No definition available. + Details
Name: | gcbc_HeldToMaturitySecuritiesInUnrealizedLossesPositionsQualitativeDisclosureNumberOfSecurityPositions
Namespace Prefix: | gcbc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- 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

X
- References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionQualitativeDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-5 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsNumberOfPositions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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/exampleRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479081/326-30-55-8 Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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 Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5A -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5A + Details
Name: | us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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

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

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

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

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