CapitalContributionsMade

No definition available.

Name:	grbk_CapitalContributionsMade
Namespace Prefix:	grbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Margin recognized on contracts with employees.

+ References

No definition available.

+ Details

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

- Definition

Cost of Land and Lots Sales

+ References

No definition available.

+ Details

Name:	grbk_CostofLandandLotsSales
Namespace Prefix:	grbk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Equity Method Investment, Board Seats

+ References

No definition available.

+ Details

Name:	grbk_EquityMethodInvestmentBoardSeats
Namespace Prefix:	grbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Equity Method Investment, Board Seats Held

+ References

No definition available.

+ Details

Name:	grbk_EquityMethodInvestmentBoardSeatsHeld
Namespace Prefix:	grbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Real Estate Properties, Remaining to be Purchased

+ References

No definition available.

+ Details

Name:	grbk_NumberofRealEstatePropertiesRemainingtobePurchased
Namespace Prefix:	grbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Payments for Capital Contributions, Percent of Total Contributions

+ References

No definition available.

+ Details

Name:	grbk_PaymentsforCapitalContributionsPercentofTotalContributions
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

PercentofCentreLivingOperationsConsolidated

+ References

No definition available.

+ Details

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

- Definition

The current amount of expenditures for a real estate project that has not yet been completed.

+ References

+ Details

Name:	us-gaap_DevelopmentInProcess
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of obligations due all related parties.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.17)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15(3),(4))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

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

- Definition

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which is directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent (that is, noncontrolling interest, previously referred to as minority interest).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(24))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(22))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

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

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of units in a real estate property owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfUnitsInRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717

+ Details

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

- Definition

Cash outflow related to amounts given in advance to show or confirm an intention to complete an acquisition of land, buildings, other structures, or any item classified as real estate.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsForDepositsOnRealEstateAcquisitions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow from the acquisition of residential property intended to generate income for the owner.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsToAcquireResidentialRealEstate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from earnings received from real estate partnerships.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

+ Details

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

- Definition

Cash received for the sale of real estate that is not part of an investing activity during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

+ Details

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

- Definition

Amount of deposits held by the entity for a related party (entity, shareholder, employee).

+ References

+ Details

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

- Definition

Amount of transactions with related party during the financial reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39622-107864

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=grbk_SuwaneeStationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=grbk_CentreLivingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=grbk_RichardA.CostelloMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=grbk_JedDolsonMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesInventoryAxis=grbk_DevelopedLotsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_InvesteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=grbk_TheParcatCogburnMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=grbk_AcademyStreetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=grbk_PartnershipDistributionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=grbk_DunwoodyTowneshipMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_ChiefFinancialOfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesInventoryAxis=grbk_TownhomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_OfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=grbk_OfficeSpaceLeaseAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=grbk_GHOHomesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=grbk_TitleClosingServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions (Details)	1 Months Ended			3 Months Ended			5 Months Ended	9 Months Ended		12 Months Ended		31 Months Ended
Related Party Transactions (Details)	Feb. 28, 2017 USD ($)	Sep. 30, 2016	Jun. 30, 2016 USD ($) lot townhome	Sep. 30, 2018 USD ($) board_seat	Dec. 31, 2017 USD ($) lot	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) board_seat	Sep. 30, 2018 USD ($) board_seat	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) lot	Dec. 31, 2015 USD ($)	Sep. 30, 2018 USD ($) board_seat	Mar. 31, 2016 lot townhome	Sep. 30, 2015 lot townhome
Related Party Transaction [Line Items]
Noncontrolling interests				$ 14,508,000	$ 16,691,000		$ 14,508,000	$ 14,508,000		$ 16,691,000		$ 14,508,000
Board seats available				3			3	3				3
Suwanee Station
Related Party Transaction [Line Items]
Board seats held \| board_seat				2			2	2				2
Board seats available \| board_seat				3			3	3				3
Centre Living
Related Party Transaction [Line Items]
Ownership percentage by parent				50.00%			50.00%	50.00%				50.00%
Ownership percentage by noncontrolling owners				50.00%			50.00%	50.00%				50.00%
Percentage of voting interest				51.00%			51.00%	51.00%				51.00%
Percent of centre living operations consolidated								100.00%
Centre Living \| Richard A. Costello
Related Party Transaction [Line Items]
Proceeds from sale of developed lot					0
Related party deposit liabilities	$ 0
Centre Living \| Jed Dolson
Related Party Transaction [Line Items]
Proceeds from sale of developed lot					0
Related party deposit liabilities	$ 0
Centre Living \| Developed Lots
Related Party Transaction [Line Items]
Number of real estate properties			1
Proceeds from sale of developed lot			$ 400,000
Cost of developed lot sold			$ 300,000
Contract price employee discount premium		13.00%
Development in Process					$ 600,000					$ 600,000
Investee \| The Parc at Cogburn
Related Party Transaction [Line Items]
Number of real estate properties \| lot														11
Number of units in real estate property \| townhome														19
Payments to acquire real estate						$ 0			$ 1,000,000		$ 1,800,000
Number of lots remaining to be purchased \| lot					0					0
Investee \| Academy Street
Related Party Transaction [Line Items]
Ownership percentage by parent													80.00%
Ownership percentage by noncontrolling owners													20.00%
Number of real estate properties \| lot													83
Capital contributions made						0		$ 0	400,000			$ 11,700,000
Distributions received				$ 900,000		900,000		2,700,000	$ 5,800,000
Payment for capital contributions, percent paid by the Company									80.00%
Investee \| Academy Street \| Partnership Distributions
Related Party Transaction [Line Items]
Amount of transaction				1,100,000		1,100,000		3,300,000	$ 7,300,000
Investee \| Suwanee Station
Related Party Transaction [Line Items]
Number of real estate properties \| townhome													73
Capital contributions made						300,000		0	400,000			2,500,000
Distributions received								400,000	200,000
Contributions to the partnership contributed by the Company						400,000			200,000
Investee \| Suwanee Station \| Parent Company
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage													50.00%
Investee \| Suwanee Station \| Affiliated Entity
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage													50.00%
Investee \| Suwanee Station \| Partnership Distributions
Related Party Transaction [Line Items]
Amount of transaction				200,000		$ 0		900,000	400,000
Investee \| Dunwoody Towneship
Related Party Transaction [Line Items]
Number of real estate properties \| lot			14		26					26
Number of units in real estate property \| townhome			40
Payments to acquire real estate									$ 2,300,000	$ 3,300,000
Chief Financial Officer \| Townhome
Related Party Transaction [Line Items]
Contract price employee discount premium	13.00%
Named Executive Officers \| Townhome
Related Party Transaction [Line Items]
Contract price employee discount premium	13.00%
Affiliated Entity \| Office Space Lease Agreements \| GHO Homes
Related Party Transaction [Line Items]
Rent expense incurred							$ 0
Amounts due to affiliates				0			0	0				0
Affiliated Entity \| Title Closing Services \| GHO Homes
Related Party Transaction [Line Items]
Amounts due to affiliates				$ 0			$ 0	$ 0				$ 0