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Fair Value Measurements (Details) - USD ($) $ in Millions | Jan. 31, 2021 | Jul. 31, 2020
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Fair Value Disclosures [Abstract] |  |
Fair value of investments in hedge funds | $ 3.5 | $ 4.8

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

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