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Accumulated Other Comprehensive Loss (Details Textual) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Oct. 31, 2017
Oct. 31, 2016
Jul. 31, 2017
Schedule of accumulated balances for each classification of other comprehensive income (loss)      
Net other comprehensive income (loss) attributable to IDT Corporation $ (398) $ (2,885)  
Unrealized gain (loss) 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 (30)    
Unrealized gain (loss) 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   $ 2,100