DERIVATIVE FINANCIAL INSTRUMENTS - Summary of the Interest Rate Swaps Outstanding (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Interest rate swaps \| Customer
Derivative [Line Items]
Maturity	3 years 10 months 25 days	2 years 9 months 18 days
Interest rate swaps \| Customer \| Minimum
Derivative [Line Items]
Fixed Rate	4.38%	2.19%
Interest rate swaps \| Customer \| Maximum
Derivative [Line Items]
Fixed Rate	6.94%	5.72%
Interest rate swaps \| Customer \| LIBOR \| Minimum
Derivative [Line Items]
Floating Rate	3.00%	0.00%
Interest rate swaps \| Customer \| LIBOR \| Maximum
Derivative [Line Items]
Floating Rate	4.25%	4.25%
Interest rate swaps \| Correspondent
Derivative [Line Items]
Maturity	3 years 10 months 25 days	2 years 9 months 18 days
Interest rate swaps \| Correspondent \| Minimum
Derivative [Line Items]
Fixed Rate	4.38%	2.19%
Interest rate swaps \| Correspondent \| Maximum
Derivative [Line Items]
Fixed Rate	6.94%	5.72%
Interest rate swaps \| Correspondent \| LIBOR \| Minimum
Derivative [Line Items]
Floating Rate	3.00%	0.00%
Interest rate swaps \| Correspondent \| LIBOR \| Maximum
Derivative [Line Items]
Floating Rate	4.25%	4.25%
Interest rate swaps \| Not Designated as Hedging Instrument \| Customer
Derivative [Line Items]
Derivative asset, Notional Amount	$ 159,205	$ 173,304
Derivative asset, Fair Value	2,515	880
Interest rate swaps \| Not Designated as Hedging Instrument \| Correspondent
Derivative [Line Items]
Derivative liability, Notional Amount	159,205	173,304
Derivative liability, Fair Value	$ (2,564)	$ (901)
Interest rate caps and collars \| Customer
Derivative [Line Items]
Maturity	1 year 2 months 12 days	1 year 8 months 12 days
Interest rate caps and collars \| Customer \| Minimum
Derivative [Line Items]
Fixed Rate	4.55%	4.55%
Interest rate caps and collars \| Customer \| Maximum
Derivative [Line Items]
Fixed Rate	6.00%	6.00%
Interest rate caps and collars \| Customer \| LIBOR \| Minimum
Derivative [Line Items]
Floating Rate	3.00%	3.00%
Interest rate caps and collars \| Customer \| LIBOR \| Maximum
Derivative [Line Items]
Floating Rate	3.75%	3.75%
Interest rate caps and collars \| Correspondent
Derivative [Line Items]
Maturity	1 year 2 months 12 days	1 year 8 months 12 days
Interest rate caps and collars \| Correspondent \| Minimum
Derivative [Line Items]
Fixed Rate	4.55%	4.55%
Interest rate caps and collars \| Correspondent \| Maximum
Derivative [Line Items]
Fixed Rate	6.00%	6.00%
Interest rate caps and collars \| Correspondent \| LIBOR \| Minimum
Derivative [Line Items]
Floating Rate	3.00%	3.00%
Interest rate caps and collars \| Correspondent \| LIBOR \| Maximum
Derivative [Line Items]
Floating Rate	3.75%	3.75%
Interest rate caps and collars \| Not Designated as Hedging Instrument \| Customer
Derivative [Line Items]
Derivative asset, Notional Amount	$ 37,030	$ 37,466
Derivative asset, Fair Value	185	75
Interest rate caps and collars \| Not Designated as Hedging Instrument \| Correspondent
Derivative [Line Items]
Derivative liability, Notional Amount	37,030	37,466
Derivative liability, Fair Value	$ (185)	$ (75)

Nominal or face amount used to calculate payments on the derivative asset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

+ Details

Name:	us-gaap_DerivativeAssetNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageRemainingMaturity1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeBasisSpreadOnVariableRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226006-175313

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226006-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Nominal or face amount used to calculate payments on the derivative liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeLiabilityNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=gnbc_CustomerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByIndustryOfCounterpartyTypeAxis=gnbc_CorrespondentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: