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Commitments and Contingencies (Narrative) (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Operating Leased Assets [Line Items]    
Loss contingency accruals $ 0us-gaap_LossContingencyAccrualAtCarryingValue $ 0us-gaap_LossContingencyAccrualAtCarryingValue
Loss Contingency, Pending Claims, Number 0us-gaap_LossContingencyPendingClaimsNumber  
Minimum    
Operating Leased Assets [Line Items]    
Estimated accrual amount per home closed 250grbk_StandardProductWarrantyAccrualEstimatedAccrualAmountPerHomeClosed
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		  
Maximum    
Operating Leased Assets [Line Items]    
Estimated accrual amount per home closed $ 800grbk_StandardProductWarrantyAccrualEstimatedAccrualAmountPerHomeClosed
/ us-gaap_RangeAxis
= us-gaap_MaximumMember