Loans - Acquired Loans (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Credit impaired acquired loans:
Outstanding principal balance	$ 7,993		$ 8,477
Recorded investment	7,099		7,498
Discount, net	894		979
Other acquired loans:
Outstanding principal balance	10,218		16,187
Recorded investment	10,079		15,900
Discount, net	139		287
Total acquired loans:
Outstanding principal balance	18,211		24,664
Recorded investment	17,178		23,398
Discount, net	1,033		1,266
Changes in the accretable yield for credit impaired acquired loans
Beginning balance	603	510
Reclassifications from nonaccretable yield	48	95
Accretion	(84)	(58)
Balance at period end	$ 567	$ 547

Represents the discount or writedown applied to loans acquired by the reporting entity.

No definition available.

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Data Type:	xbrli:monetaryItemType
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- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities excluding loans with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer.

+ References

No definition available.

+ Details

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- Definition

The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans not accounted for as debt securities, excluding loans with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer.

+ References

No definition available.

+ Details

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- Definition

Represents the discount or writedown applied to a loan acquired by the reporting entity excluding loans acquired with deteriorated credit quality since origination.

+ References

No definition available.

+ Details

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Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Represents the outstanding principal balance of loans acquired in a transfer.

+ References

No definition available.

+ Details

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- Definition

Represents the discount or writedown applied to a loan acquired with deteriorated credit quality.

+ References

No definition available.

+ Details

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Namespace Prefix:	gnbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the recorded investment in loans acquired in a transfer.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

Amount of the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 15

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

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- Definition

Amount of decrease from accretion of the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

+ References

+ Details

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- Definition

Amount of reclassification (to) from nonaccretable difference (from) to the excess of a loan's cash flows expected to be collected over the investor's initial investment in acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

+ References

+ Details

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- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 15

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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- Definition

The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 15

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

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