The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

No definition available.

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

+ References

No definition available.

+ Details

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

- Definition

Commitment and contingencies.

+ References

No definition available.

+ Details

Name:	zgh_CommitmentAndContingenciesLineItems
Namespace Prefix:	zgh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The Period of purchase commitment for tele communication services.

+ References

No definition available.

+ Details

Name:	zgh_PeriodOfPurchaseCommitmentForTeleCommunicationServices
Namespace Prefix:	zgh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of purchase commitment for tele Communication services during the period.

+ References

No definition available.

+ Details

Name:	zgh_PurchaseCommitmentForTeleCommunicationServices
Namespace Prefix:	zgh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LongtermDebtTypeAxis=zgh_CreditLetterAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

COMMITMENTS AND CONTINGENCIES (Narrative) (Details) - 3 months ended Sep. 30, 2018 $ in Millions, $ in Millions	USD ($)	CAD ($)	USD ($)
Commitment And Contingencies [Line Items]
Purchase commitments			$ 625.6
Purchase commitments - tele communication services	$ 40.0
Purchase commitment period	3 years
Outstanding letters of credit			8.1
Credit Letter Agreement [Member]
Commitment And Contingencies [Line Items]
Outstanding letters of credit		$ 3.5	2.7
Credit letter agreement limit		$ 5.0	$ 3.9