The percentage of increase in cash dividend rate during the period.

No definition available.

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

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Date the dividend to be paid was declared, in CCYY-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 CCYY-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

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

- Details

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Namespace Prefix:
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Subsequent events (Details) - $ / shares		12 Months Ended
Subsequent events (Details) - $ / shares	Jul. 18, 2018	Jun. 30, 2018	Jun. 30, 2017
Dividends [Abstract]
Common stock, dividends declared (in dollars per share)		$ 0.39	$ 0.38
2017 Annual Dividend [Member]
Dividends [Abstract]
Common stock, dividends declared (in dollars per share)		$ 0.39
Subsequent Event [Member] \| 2018 Q4 Dividend [Member]
Dividends [Abstract]
Common stock, dividends declared (in dollars per share)	$ 0.10
Percentage of increase in cash dividend rate	2.60%
Dividends payable, date of record	Aug. 15, 2018
Dividends payable, date paid	Aug. 31, 2018
Dividends payable, date declared	Jul. 18, 2018
Subsequent Event [Member] \| 2018 Annual Dividend [Member]
Dividends [Abstract]
Common stock, dividends declared (in dollars per share)	$ 0.40