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Income Taxes (Narrative) (Details) (USD $)
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Oct. 27, 2014
Operating Loss Carryforwards [Line Items]        
Uncertain income tax positions $ 0us-gaap_UnrecognizedTaxBenefits      
Basis in partnerships 26,123,000grbk_DeferredTaxAssetsBasisinPartnership 0grbk_DeferredTaxAssetsBasisinPartnership    
One-time income tax benefit 25,244,000grbk_EffectiveIncomeTaxRateReconciliationChangeinTaxStatusAmount 0grbk_EffectiveIncomeTaxRateReconciliationChangeinTaxStatusAmount 0grbk_EffectiveIncomeTaxRateReconciliationChangeinTaxStatusAmount  
Deferred tax assets 89,239,000us-gaap_DeferredTaxAssetsNet 0us-gaap_DeferredTaxAssetsNet    
Deferred tax asset related to net operating loss carryforwards 62,575,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic 0us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic    
Valuation allowance for deferred tax assets 1,161,000us-gaap_DeferredTaxAssetsValuationAllowance 0us-gaap_DeferredTaxAssetsValuationAllowance 0us-gaap_DeferredTaxAssetsValuationAllowance  
Federal        
Operating Loss Carryforwards [Line Items]        
Net operating loss carryforward 178,800,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
     
Operating loss carryforward, expiration date Dec. 31, 2029      
State        
Operating Loss Carryforwards [Line Items]        
Net operating loss carryforward 21,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
     
Valuation allowance related to state loss carryforwards 21,600,000us-gaap_OperatingLossCarryforwardsValuationAllowance
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
     
Reverse Recapitalization        
Operating Loss Carryforwards [Line Items]        
One-time income tax benefit       26,564,000grbk_EffectiveIncomeTaxRateReconciliationChangeinTaxStatusAmount
/ us-gaap_BusinessAcquisitionAxis
= grbk_ReverseRecapitalizationMember
Deferred tax assets       63,900,000us-gaap_DeferredTaxAssetsNet
/ us-gaap_BusinessAcquisitionAxis
= grbk_ReverseRecapitalizationMember
Reverse Recapitalization | Federal        
Operating Loss Carryforwards [Line Items]        
Net operating loss carryforward       182,300,000us-gaap_OperatingLossCarryforwards
/ us-gaap_BusinessAcquisitionAxis
= grbk_ReverseRecapitalizationMember
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
Reverse Recapitalization | State        
Operating Loss Carryforwards [Line Items]        
Net operating loss carryforward       $ 21,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_BusinessAcquisitionAxis
= grbk_ReverseRecapitalizationMember
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember