LOANS - Acquired Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Credit impaired acquired loans:
Outstanding principal balance	$ 7,621	$ 18,498
Recorded investment	7,037	15,965
Discount, net	584	2,533
Other acquired loans:
Outstanding principal balance	282,476	499,939
Deferred fees, net	(33)	(72)
Recorded investment	281,743	498,029
Discount, net	700	1,838
Total acquired loans:
Outstanding principal balance	290,097	518,437
Deferred fees, net	(33)	(72)
Recorded investment	288,780	513,994
Discount, net	$ 1,284	$ 4,371

Represents the deferred fee net on loans acquired by the reporting entity.

No definition available.

Name:	gnbc_AcquiredLoansDeferredFees
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the discount or writedown applied to loans acquired by the reporting entity.

+ References

No definition available.

+ Details

Name:	gnbc_AcquiredLoansDiscount
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities excluding loans with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer.

+ References

No definition available.

+ Details

Name:	gnbc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesCarryingAmountNetOther
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans not accounted for as debt securities, excluding loans with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer.

+ References

No definition available.

+ Details

Name:	gnbc_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesOutstandingBalanceOther
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the discount or writedown applied to a loan acquired by the reporting entity excluding loans acquired with deteriorated credit quality since origination.

+ References

No definition available.

+ Details

Name:	gnbc_OtherAcquiredLoansDiscount
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the outstanding principal balance of loans acquired in a transfer.

+ References

No definition available.

+ Details

Name:	gnbc_OutstandingPrincipalBalanceLoansAcquired
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the discount or writedown applied to a loan acquired with deteriorated credit quality.

+ References

No definition available.

+ Details

Name:	gnbc_PurchasedCreditImpairedLoansDiscount
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the recorded investment in loans acquired in a transfer.

+ References

No definition available.

+ Details

Name:	gnbc_RecordedInvestmentLoansAcquired
Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

+ Details

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

- Definition

The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

+ Details

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

- References

No definition available.

+ Details

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