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Fair Value Measurements (Details) - USD ($) $ in Millions	Jul. 31, 2020	Jul. 31, 2019
Fair Value Disclosures [Abstract]
Fair value of investments in hedge funds	$ 4.8	$ 5.5

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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

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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