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Employee Benefits - Stock Appreciation Rigthts Plan (Details) (SAR Plan, Stock Appreciation Rights (SARs) [Member], USD $)
In Thousands, except Share data, unless otherwise specified
0 Months Ended 3 Months Ended
May 18, 2007
Mar. 31, 2015
Dec. 31, 2014
SAR Plan | Stock Appreciation Rights (SARs) [Member]
     
Employee Benefits      
Number of units authorized for issuance 200,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
/ us-gaap_PlanNameAxis
= gnbc_SarPlanMember
   
Term of awards 10 years    
Units outstanding   121,334us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
/ us-gaap_PlanNameAxis
= gnbc_SarPlanMember
134,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
/ us-gaap_PlanNameAxis
= gnbc_SarPlanMember
Stock-based compensation exepense, reversal   $ 108gnbc_AllocatedShareBasedCompensationExpenseReversal
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember
/ us-gaap_PlanNameAxis
= gnbc_SarPlanMember