Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, July 31, 2016			$ (3,744)
Other comprehensive income (loss) attributable to IDT Corporation before reclassifications			(2,161)
Net other comprehensive loss attributable to IDT Corporation	$ 419	$ (4,154)	(2,466)	$ (3,515)
Balance, January 31, 2017	(6,210)		(6,210)
Unrealized Gain (Loss) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, July 31, 2016			8
Other comprehensive income (loss) attributable to IDT Corporation before reclassifications			242
Net other comprehensive loss attributable to IDT Corporation			(63)
Balance, January 31, 2017	(55)		(55)
Unrealized Gain (Loss) on Available-for-Sale Securities [Member] | Other (expense) income, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: reclassification for gain included in net income			(305)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, July 31, 2016			(3,752)
Other comprehensive income (loss) attributable to IDT Corporation before reclassifications			(2,403)
Net other comprehensive loss attributable to IDT Corporation			(2,403)
Balance, January 31, 2017	(6,155)		(6,155)
Foreign Currency Translation [Member] | Other (expense) income, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: reclassification for gain included in net income
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, July 31, 2016			(3,744)
Other comprehensive income (loss) attributable to IDT Corporation before reclassifications			(2,161)
Net other comprehensive loss attributable to IDT Corporation			(2,466)
Balance, January 31, 2017	$ (6,210)		(6,210)
Accumulated Other Comprehensive Loss [Member] | Other (expense) income, net [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: reclassification for gain included in net income			$ (305)