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Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Jul. 31, 2018	Jul. 31, 2017	Jul. 31, 2016
Defined Contribution Plans (Textual)
Maximum percentage of participants contribution	20.00%
Percentage of discretionary matching contributions	50.00%
Defined benefit plan compensation	6.00%
Company's cost for contributions to the plan	$ 1.1	$ 1.2	$ 1.4
Employment period contributions, description	First five years.
401(k) Plan [Member]
Defined Contribution Plans (Textual)
Class B common stock to the plan for matching contributions			94,712

X

- Definition

Employment period contributions description.

+ References

No definition available.

+ Details

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Namespace Prefix:	idt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Defined Contribution Plan Employer Matching Contribution In Employer Shares.

+ References

No definition available.

+ Details

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Namespace Prefix:	idt_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Defined contribution plan employer matching contribution limit percent.

+ References

No definition available.

+ Details

Name:	idt_DefinedContributionPlanEmployerMatchingContributionLimitPercent
Namespace Prefix:	idt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	idt_DefinedContributionPlansTextualAbstract
Namespace Prefix:	idt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

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

X

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RetirementPlanNameAxis=idt_DefinedContributionPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: