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Investment in Direct Finance Leases (Details) | 12 Months Ended
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Dec. 31, 2014
Leases [Abstract] |
Direct finance lease, interest rate, minimum | 10.00%grbk_CapitalLeasesNetInvestmentinDirectFinancingLeasesInterestRateMinimum
Direct finance lease, interest rate, maximum | 12.00%grbk_CapitalLeasesNetInvestmentinDirectFinancingLeasesInterestRateMaximum

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- Definition Capital Leases, Net Investment in Direct Financing Leases, Interest Rate Maximum + References No definition available. + Details
Name: | grbk_CapitalLeasesNetInvestmentinDirectFinancingLeasesInterestRateMaximum
Namespace Prefix: | grbk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Capital Leases, Net Investment in Direct Financing Leases, Interest Rate + References No definition available. + Details
Name: | grbk_CapitalLeasesNetInvestmentinDirectFinancingLeasesInterestRateMinimum
Namespace Prefix: | grbk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LeasesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration