Fair Value Measurements (Details 1) - Level 3 [Member] - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2019	Apr. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance, beginning of period	$ 2,745	$ 6,300		$ 6,300
Transfer into Level 3 from adoption of change in accounting for equity investments			2,794
Rafael Spin-Off		(6,300)		(6,300)
Total gains recognized in "Other income (expense), net"	599		550
Balance, end of period	3,344		3,344
Change in unrealized gains or losses for the period included in earnings for assets held at the end of the period	$ 599		$ 550

Amount of change in unrealized gains or losses included in earnings for assets held at end of period.

No definition available.

Name:	idt_AmountOfChangeInUnrealizedGainsOrLossesIncludedInEarningsForAssetsHeldAtEndOfPeriod
Namespace Prefix:	idt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of fair value measurement with unobservable inputs reconciliation recurring basis asset rafael spin-off.

+ References

No definition available.

+ Details

Name:	idt_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSpinOff
Namespace Prefix:	idt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of fair value measurement with unobservable inputs reconciliation recurring basis asset transfers into level three from adoption of accounting change.

+ References

No definition available.

+ Details

Name:	idt_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevelThreeFromAdoptionOfAccountingChange
Namespace Prefix:	idt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: