Accumulated Other Comprehensive Loss (Details Textual) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2018	Apr. 30, 2017	Jul. 31, 2017
Accumulated Other Comprehensive Loss (Textual)
Net other comprehensive income (loss) attributable to IDT Corporation	$ 204	$ 1,159	$ 16	$ (1,307)
Unrealized gain (loss) on available-for-sale securities [Member]
Accumulated Other Comprehensive Loss (Textual)
Net other comprehensive income (loss) attributable to IDT Corporation			$ (122)
Unrealized gain (loss) on available-for-sale securities [Member] | Rafael Pharmaceuticals, Inc. Series D Note [Member]
Accumulated Other Comprehensive Loss (Textual)
Net other comprehensive income (loss) attributable to IDT Corporation					$ 2,100