Loans - Impaired Loans Summary (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Impaired loans summary
Nonaccrual loans	$ 2,127,000	$ 1,496,000
Accruing loans past due 90 days or more	16,000	1,316,000
Restructured loans - nonaccrual	2,717,000gnbc_NonaccrualLoansRestructured	9,864,000gnbc_NonaccrualLoansRestructured
Restructured loans - accruing	2,257,000gnbc_RestructuredLoansAccruing	4,072,000gnbc_RestructuredLoansAccruing
Total nonperforming loans	7,117,000gnbc_NonperfomringLoans	16,748,000gnbc_NonperfomringLoans
Impaired loans, other infomation
Impaired loans, allowance	468,000	4,335,000
Impaired loans, average recorded investment	11,700,000	38,000,000
Nonaccrual loans, interest recognized	667,000	373,000	253,000
Total nonaccrual loans	4,800,000	11,400,000
Loans and Leases Receivable, Impaired, Interest Lost on Nonaccrual Loans	$ 401,000	$ 657,000	$ 775,000

Represents the balance of nonaccrual loans excluding nonaccrual restructured loans.

No definition available.

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- Definition

Represents the balance of nonaccrual loans that have been restructured.

+ References

No definition available.

+ Details

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- Definition

Represents all of the loans classified as nonperforming by the reporting entity, including but not limited to nonaccrual loans, accruing loans past due 90 days or more, and accruing and nonaccruing restructured loans.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Represents the balance of restructured loans that are still accruing.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Recorded investment in financing receivables that are 90 days or more past due and still accruing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5111-111524

+ Details

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- Definition

Average 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 15

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

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- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of interest income on impaired and nonperforming loans that is due to the change in present value attributable to the passage of time.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5278-111524

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- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

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- Definition

Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5111-111524

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