Loans - Acquired Loans (Details 5) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Credit impaired acquired loans:
Outstanding principal balance	$ 15,417		$ 16,224
Recorded investment	13,409		14,154
Discount, net	2,008		2,070
Other acquired loans:
Outstanding principal balance	202,804		226,284
Deferred fees, net	(58)gnbc_OtherAcquiredLoansDeferredFees		(3)gnbc_OtherAcquiredLoansDeferredFees
Recorded investment	201,280		224,270
Discount, net	1,466gnbc_OtherAcquiredLoansDiscount		2,011gnbc_OtherAcquiredLoansDiscount
Total acquired loans:
Outstanding principal balance	218,221		242,508
Deferred fees, net	(58)gnbc_AcquiredLoansDeferredFees		(3)gnbc_AcquiredLoansDeferredFees
Recorded investment	214,689gnbc_RecordedInvestmentLoansAcquired		238,424gnbc_RecordedInvestmentLoansAcquired
Discount, net	3,474gnbc_AcquiredLoansDiscount		4,081gnbc_AcquiredLoansDiscount
Changes in the accretable yield for credit impaired acquired loans
Beginning balance	685	603
Additions		62
Reclassifications from nonaccretable yield	(480)	(161)
Accretion	(63)	(141)
Balance at period end	$ 1,102	$ 685

Represents the deferred fee net on loans acquired by the reporting entity.

No definition available.

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- Definition

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

+ References

No definition available.

+ 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 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 deferred fee on other loans acquired by the reporting entity.

+ 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_
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- 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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- Definition

Represents the recorded investment in loans acquired in a transfer.

+ References

No definition available.

+ Details

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- 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

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-Publisher FASB

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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

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- Definition

Amount of additions 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

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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

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-Publisher FASB

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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

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-Name Accounting Standards Codification

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-Paragraph 2

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

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