Loans - Troubled Debt Restructurings (Details) (USD $)	6 Months Ended
Jun. 30, 2014 contract	Jun. 30, 2013 contract
Financing Receivable, Modifications [Line Items]
Number of contracts	5	9
Pre-modification outstanding recorded investment	$ 700,000	$ 10,092,000
Recorded investment	697,000	8,789,000
Allowance for loan losses, troubled debt restructurings	1,700,000
Troubled debt restructuring	11,000,000
Commercial & Industrial [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	3	5
Pre-modification outstanding recorded investment	545,000	2,737,000
Recorded investment	542,000	2,776,000
Commerical real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts		1
Pre-modification outstanding recorded investment		2,562,000
Recorded investment		2,562,000
Construction, land & land development [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	1	2
Pre-modification outstanding recorded investment	30,000	4,671,000
Recorded investment	30,000	3,329,000
Other consumer [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	1	1
Pre-modification outstanding recorded investment	125,000	122,000
Recorded investment	$ 125,000	$ 122,000

Represents the amount of loans that have been modified in a troubled debt restructuring.

No definition available.

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

- Definition

Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

+ Details

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