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Income Taxes (Rollforward of Valuation Allowances) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Income Tax Disclosure [Abstract]    
Valuation allowance at beginning of the year $ 0us-gaap_DeferredTaxAssetsValuationAllowance $ 0us-gaap_DeferredTaxAssetsValuationAllowance
BioFuel valuation allowance at the Transaction Date 65,020grbk_ValuationAllowanceDeferredTaxAssetIncreaseAmount 0grbk_ValuationAllowanceDeferredTaxAssetIncreaseAmount
Release of valuation allowance at Transaction Date (63,859)grbk_ValuationAllowanceDeferredTaxAssetDecreaseAmount 0grbk_ValuationAllowanceDeferredTaxAssetDecreaseAmount
Valuation allowance at end of the year $ 1,161us-gaap_DeferredTaxAssetsValuationAllowance $ 0us-gaap_DeferredTaxAssetsValuationAllowance