Accumulated Other Comprehensive Income (Loss) (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Jul. 31, 2017		Jul. 31, 2016	Jul. 31, 2015
Schedule of accumulated balances for each classification of other comprehensive income (loss)
Net other comprehensive income (loss) attributable to IDT Corporation	$ 1,401		$ (5,544)	$ (2,999)
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	2,126	[1]	$ 583
Unrealized gain (loss) 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 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

[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.