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Accumulated Other Comprehensive Loss (Details Textual) - USD ($)
$ in Thousands
 12 Months Ended
Jul. 31, 2018
Jul. 31, 2017
Jul. 31, 2016
Schedule of accumulated balances for each classification of other comprehensive income (loss)      
Net other comprehensive income (loss) attributable to IDT Corporation $ (359) $ 1,401 $ (5,544)
Unrealized (loss) gain on available-for-sale securities [Member]      
Schedule of accumulated balances for each classification of other comprehensive income (loss)      
Net other comprehensive income (loss) attributable to IDT Corporation $ (177) 2,126 [1] $ 583
Unrealized (loss) gain on available-for-sale securities [Member] | Marketable Securities [Member]      
Schedule of accumulated balances for each classification of other comprehensive income (loss)      
Net other comprehensive income (loss) attributable to IDT Corporation   26  
Unrealized (loss) gain on available-for-sale securities [Member] | Rafael Pharmaceuticals, Inc. [Member]      
Schedule of accumulated balances for each classification of other comprehensive income (loss)      
Net other comprehensive income (loss) attributable to IDT Corporation   $ 2,100  
[1] In fiscal 2017, net other comprehensive income attributable to IDT Corporation from unrealized gains on available-for-sale securities included unrealized gains on the Rafael convertible promissory notes of $2.1 million and unrealized gains, net on marketable securities of $26,000.