Employee Benefit Plans, Defined Contribution Plan (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2017	Jun. 30, 2016
Defined Contribution Plan Disclosure [Line Items]
Required service period to participate in Defined Contribution Plan	3 months
Equal matching of employer contribution dollar for dollar	3.00%
Percentage of employer contribution for next 3%	50.00%
Company contributions associated with the contribution plan	$ 255	$ 246
Maximum [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of compensation allowed to be contributed by employees on pre-tax basis	50.00%

Represents the percentage of compensation allowed to be contributed by employees on a pre-tax basis.

No definition available.

Name:	gcbc_DefinedContributionPlanPercentageOfCompensationAllowedToBeContributedByEmployeeOnPreTaxBasis
Namespace Prefix:	gcbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the equal matching of employer contribution dollar for dollar.

+ References

No definition available.

+ Details

Name:	gcbc_EqualMatchingOfEmployerContributionDollarForDollar
Namespace Prefix:	gcbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of employer contribution for next specified percentage.

+ References

No definition available.

+ Details

Name:	gcbc_PercentageOfEmployerContributionForNextSpecifiedPercentage
Namespace Prefix:	gcbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Element represents the required service period after which an employee can participate in the defined contribution plan, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	gcbc_RequiredServicePeriodToParticipateInDefinedContributionPlan
Namespace Prefix:	gcbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: