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Equity Investments (Details 1) - USD ($) $ in Thousands | 3 Months Ended
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Oct. 31, 2018 | Oct. 31, 2017
Equity Securities, FV-NI, Gain (Loss) [Abstract] |  |
Net (losses) gains recognized during the period on equity investments | $ (46) | $ 20
Less: net gains and losses recognized during the period on equity investments sold during the period |  |
Unrealized (losses) gains recognized during the period on equity investments still held at the reporting date | $ (46) | $ 20

X
- Definition Amount of recognized gain (loss) net on equity securities (FV-NI). + References No definition available. + Details
Name: | idt_EquitySecuritiesFvNiRecognizedGainLossNet
Namespace Prefix: | idt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of recognized gain (loss) net on equity securities which has been sold (FV-NI). + References No definition available. + Details
Name: | idt_EquitySecuritiesFvNiRecognizedGainLossNetOnEquitySecuritiesSold
Namespace Prefix: | idt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of recognized unrealized gain (loss) net on equity securities held (FV-NI). + References No definition available. + Details
Name: | idt_EquitySecuritiesFvNiRecognizedUnrealizedGainLossNetEquitySecuritiesHeld
Namespace Prefix: | idt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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