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Defined Contribution Plans (Details) - USD ($) shares in Thousands, $ in Millions | 12 Months Ended
--- | ---
Jul. 31, 2017 | Jul. 31, 2016 | Jul. 31, 2015
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 | $ 1.2 | $ 1.4 | $ 1.3
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 | 70,843

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- 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

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- 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
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- 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

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- References No definition available. + Details
Name: | idt_DefinedContributionPlansTextualAbstract
Namespace Prefix: | idt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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

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- 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

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