LOANS - Impaired Loans Individually Evaluated (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
With an allowance recorded:
Related allowance	$ 5,667	$ 1,775
Total:
Recorded investment	70,431	93,324
Unpaid principal balance	70,713	93,867
Related allowance	5,667	1,775
Commercial & industrial
With an allowance recorded:
Related allowance	5,409	1,654
Total:
Recorded investment	60,394	71,082
Unpaid principal balance	60,655	71,576
Related allowance	5,409	1,654
Residential mortgage
With an allowance recorded:
Recorded investment		119
Unpaid principal balance		119
Related allowance		119
Total:
Related allowance		119
Real estate
With an allowance recorded:
Related allowance	224
Total:
Recorded investment	9,764	22,023
Unpaid principal balance	9,791	22,071
Related allowance	224
Consumer and other
With an allowance recorded:
Related allowance	34	121
Total:
Recorded investment	273	219
Unpaid principal balance	267	220
Related allowance	34	121
Originated
With no related allowance recorded:
Recorded investment	36,028	41,920
Unpaid principal balance	36,208	42,178
With an allowance recorded:
Recorded investment	20,087	3,111
Unpaid principal balance	20,168	3,157
Related allowance	5,663	513
Total:
Recorded investment	56,115	45,031
Unpaid principal balance	56,376	45,335
Related allowance	5,663	513
Originated \| Commercial & industrial
With no related allowance recorded:
Recorded investment	32,794	33,415
Unpaid principal balance	32,974	33,671
With an allowance recorded:
Recorded investment	16,862	2,992
Unpaid principal balance	16,943	3,038
Related allowance	5,409	394
Total:
Related allowance	5,409	394
Originated \| Commercial real estate
With no related allowance recorded:
Recorded investment		7,155
Unpaid principal balance		7,162
Originated \| Construction, land & land development
With no related allowance recorded:
Recorded investment	111
Unpaid principal balance	111
With an allowance recorded:
Recorded investment	3,176
Unpaid principal balance	3,176
Related allowance	224
Total:
Related allowance	224
Originated \| Residential mortgage
With no related allowance recorded:
Recorded investment	3,111	1,254
Unpaid principal balance	3,111	1,249
Originated \| Consumer and other
With no related allowance recorded:
Recorded investment	12	96
Unpaid principal balance	12	96
With an allowance recorded:
Recorded investment	49
Unpaid principal balance	49
Related allowance	30
Total:
Related allowance	30
Acquired
With no related allowance recorded:
Recorded investment	14,312	46,538
Unpaid principal balance	14,333	46,755
With an allowance recorded:
Recorded investment	4	1,755
Unpaid principal balance	4	1,777
Related allowance	4	1,262
Total:
Recorded investment	14,316	48,293
Unpaid principal balance	14,337	48,532
Related allowance	4	1,262
Acquired \| Commercial & industrial
With no related allowance recorded:
Recorded investment	10,738	32,922
Unpaid principal balance	10,738	33,093
Acquired \| Owner occupied commercial real estate
With no related allowance recorded:
Recorded investment	1,886	8,942
Unpaid principal balance	1,910	8,949
Acquired \| Commercial real estate
With no related allowance recorded:
Recorded investment	312	2,002
Unpaid principal balance	312	2,026
Acquired \| Construction, land & land development
With no related allowance recorded:
Recorded investment	714	978
Unpaid principal balance	716	991
Acquired \| Residential mortgage
With no related allowance recorded:
Recorded investment	454	1,692
Unpaid principal balance	455	1,694
With an allowance recorded:
Recorded investment		1,753
Unpaid principal balance		1,774
Related allowance		1,260
Total:
Related allowance		1,260
Acquired \| Consumer and other
With no related allowance recorded:
Recorded investment	208	2
Unpaid principal balance	202	2
With an allowance recorded:
Recorded investment	4	2
Unpaid principal balance	4	3
Related allowance	4	2
Total:
Related allowance	$ 4	$ 2

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=66008989&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
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: 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=66008989&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=66008989&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

+ 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

+ 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=us-gaap_ResidentialMortgageMember
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_CommercialRealEstateMultiFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=gnbc_ConstructionLandAndLandDevelopmentMember
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:

- Details

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