Debt (Narrative) (Details) (Secured Debt, USD $) In Millions, unless otherwise specified	0 Months Ended
Oct. 27, 2014	Oct. 28, 2015
Debt Instrument [Line Items]
Maximum period electable to pay interest in kind	12 months
Reverse Recapitalization \| Term Loan Facility
Debt Instrument [Line Items]
Mandatory prepayment, percentage of net cash proceeds from incurrence of debt	100.00%
Voluntary prepayment premium	1.00%
Reverse Recapitalization \| Term Loan Facility \| Other Assets
Debt Instrument [Line Items]
Debt issuance cost	0.4
Reverse Recapitalization \| Term Loan Facility \| Greenlight Capital, Inc
Debt Instrument [Line Items]
Percentage of voting interest	49.90%
Debt instrument, term	5 years
Debt instrument, face amount	150.0
Stated interest rate	9.00%
Reverse Recapitalization \| Minimum \| Term Loan Facility
Debt Instrument [Line Items]
Fixed charge coverage ratio, one (no more than)	0.80
Fixed charge coverage ratio, two (no more than)	1.00
Reverse Recapitalization \| Maximum \| Term Loan Facility
Debt Instrument [Line Items]
Fixed charge coverage ratio, one (no more than)	1.00
Fixed charge coverage ratio, two (no more than)	1.20
Scenario, Forecast \| Reverse Recapitalization \| Term Loan Facility \| Greenlight Capital, Inc
Debt Instrument [Line Items]
Stated interest rate		10.00%

Debt Instrument, Covenant Terms, Fixed Charge Coverage Ratio, One

No definition available.

Name:	grbk_DebtInstrumentCovenantTermsFixedChargeCoverageRatio
Namespace Prefix:	grbk_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Covenant Terms, Fixed Charge Coverage Ratio, Two

+ References

No definition available.

+ Details

Name:	grbk_DebtInstrumentCovenantTermsFixedChargeCoverageRatioTwo
Namespace Prefix:	grbk_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Mandatory percentage of the net cash proceeds received from the incurrence of certain debt or the issuance of certain equity securities that should be used to prepay the term loan facility.

+ References

No definition available.

+ Details

Name:	grbk_DebtInstrumentMandatoryPrepaymentTermsPercentageofNetCashProceedsfromIncurrenceofIndebtedness
Namespace Prefix:	grbk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Maximum Period Electable to Pay Interest in Kind

+ References

No definition available.

+ Details

Name:	grbk_DebtInstrumentMaximumPeriodElectabletoPayInterestinKind
Namespace Prefix:	grbk_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Prepayment penalty on term loan facility.

+ References

No definition available.

+ Details

Name:	grbk_DebtInstrumentVoluntaryPrepaymentPremiumPercentage
Namespace Prefix:	grbk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of Voting Interest

+ References

No definition available.

+ Details

Name:	grbk_PercentageofVotingInterest
Namespace Prefix:	grbk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).

+ References

No definition available.

+ Details

Name:	us-gaap_DebtIssuanceCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=grbk_ReverseRecapitalizationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: