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Fair Value Measurements (Details Textual) - USD ($) $ in Millions | Jul. 31, 2018 | Jul. 31, 2017
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Fair Value Measurements (Textual) |  |
Fair value of investments in hedge funds | $ 4.8 | $ 8.6
Carrying value of investments | $ 3.0 | $ 10.8

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- Definition This item represents a financial instrument (as defined) for which it is not practicable to estimate fair value. Such disclosure may be for an individual financial instrument or a class of financial instruments. + References No definition available. + Details
Name: | idt_FairValueEstimateNotPracticableCostMethodInvestment
Namespace Prefix: | idt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Definition Investment in hedge funds. + References No definition available. + Details
Name: | idt_InvestmentInHedgeFunds
Namespace Prefix: | idt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant