Accumulated Other Comprehensive Loss (Details Textual) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017	Jul. 31, 2017
Accumulated Other Comprehensive Loss (Textual)
Net other comprehensive income (loss) attributable to IDT Corporation	$ 210	$ 419	$ (188)	$ (2,466)
Unrealized gain (loss) on available-for-sale securities [Member]
Accumulated Other Comprehensive Loss (Textual)
Net other comprehensive income (loss) attributable to IDT Corporation			(150)
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		$ 2,100