LOANS - Impaired Loans Individually Evaluated (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
With an allowance recorded:
Related Allowance	$ 10,368	$ 5,667
Total:
Recorded Investment	57,884	70,431
Unpaid Principal Balance	58,186	70,713
Commercial & industrial
With an allowance recorded:
Related Allowance	9,652	5,409
Total:
Recorded Investment	45,635	60,394
Unpaid Principal Balance	45,906	60,655
Real estate
With an allowance recorded:
Related Allowance	684	224
Total:
Recorded Investment	12,002	9,764
Unpaid Principal Balance	12,038	9,791
Consumer and other
With an allowance recorded:
Related Allowance	32	34
Total:
Recorded Investment	247	273
Unpaid Principal Balance	242	267
Originated
With no related allowance recorded:
Recorded Investment	22,087	36,028
Unpaid Principal Balance	22,303	36,208
With an allowance recorded:
Recorded Investment	28,365	20,087
Unpaid Principal Balance	28,428	20,168
Related Allowance	10,213	5,663
Total:
Recorded Investment	50,452	56,115
Unpaid Principal Balance	50,731	56,376
Originated \| Commercial & industrial
With no related allowance recorded:
Recorded Investment	16,215	32,794
Unpaid Principal Balance	16,423	32,974
With an allowance recorded:
Recorded Investment	24,826	16,862
Unpaid Principal Balance	24,889	16,943
Related Allowance	9,652	5,409
Originated \| Owner occupied commercial real estate
With no related allowance recorded:
Recorded Investment	1,731
Unpaid Principal Balance	1,738
With an allowance recorded:
Recorded Investment	3,494
Unpaid Principal Balance	3,494
Related Allowance	531
Originated \| Commercial real estate
With no related allowance recorded:
Recorded Investment	175
Unpaid Principal Balance	175
Originated \| Construction, land & land development
With no related allowance recorded:
Recorded Investment	107	111
Unpaid Principal Balance	108	111
With an allowance recorded:
Recorded Investment		3,176
Unpaid Principal Balance		3,176
Related Allowance		224
Originated \| Residential mortgage
With no related allowance recorded:
Recorded Investment	3,859	3,111
Unpaid Principal Balance	3,859	3,111
Originated \| Consumer and other
With no related allowance recorded:
Recorded Investment		12
Unpaid Principal Balance		12
With an allowance recorded:
Recorded Investment	45	49
Unpaid Principal Balance	45	49
Related Allowance	30	30
Acquired
With no related allowance recorded:
Recorded Investment	6,746	14,312
Unpaid Principal Balance	6,753	14,333
With an allowance recorded:
Recorded Investment	686	4
Unpaid Principal Balance	702	4
Related Allowance	155	4
Total:
Recorded Investment	7,432	14,316
Unpaid Principal Balance	7,455	14,337
Acquired \| Commercial & industrial
With no related allowance recorded:
Recorded Investment	4,594	10,738
Unpaid Principal Balance	4,594	10,738
Acquired \| Owner occupied commercial real estate
With no related allowance recorded:
Recorded Investment	1,043
Unpaid Principal Balance	1,053
With an allowance recorded:
Recorded Investment	684	1,886
Unpaid Principal Balance	699	1,910
Related Allowance	153
Acquired \| Commercial real estate
With no related allowance recorded:
Recorded Investment	105	312
Unpaid Principal Balance	105	312
Acquired \| Construction, land & land development
With no related allowance recorded:
Recorded Investment	700
Unpaid Principal Balance	702
With an allowance recorded:
Recorded Investment		714
Unpaid Principal Balance		716
Acquired \| Residential mortgage
With no related allowance recorded:
Recorded Investment	104	454
Unpaid Principal Balance	105	455
Acquired \| Consumer and other
With no related allowance recorded:
Recorded Investment	200	208
Unpaid Principal Balance	194	202
With an allowance recorded:
Recorded Investment	2	4
Unpaid Principal Balance	3	4
Related Allowance	$ 2	$ 4

No definition available.

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnbc_RealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gnbc_OriginatedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnbc_CommercialRealEstateOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnbc_CommercialRealEstateMultiFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gnbc_AcquiredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: