Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended													12 Months Ended
	Jul. 31, 2017	[1]	Apr. 29, 2017	[2]	Jan. 31, 2017	Oct. 31, 2016	[3]	Jul. 31, 2016	[4]	Apr. 30, 2016	[5]	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2017	Jul. 31, 2016	Jul. 31, 2015
Selected Quarterly Financial Data (Unaudited) [Abstract]
Revenues	$ 394,987		$ 370,035		$ 367,556	$ 369,151		$ 368,075		$ 355,154		$ 382,454	$ 390,578	$ 1,501,729	$ 1,496,261	$ 1,596,777
Direct cost of revenues	337,062		314,704		310,913	313,029		309,139		293,220		319,724	324,511	1,275,708	1,246,594	1,328,363
Income (loss) from operations	3,737		(6,502)		3,128	5,186		6,225		5,676		6,377	7,925	5,549	26,203	93,050
Net income (loss)	(9,458)		(4,452)		1,257	22,294		11,419		4,701		4,663	4,575	9,641	25,358	86,115
Net income (loss) attributable to IDT Corporation	$ (9,841)		$ (4,775)		$ 875	$ 21,918		$ 11,019		$ 4,237		$ 4,065	$ 4,193	$ 8,177	$ 23,514	$ 84,490
Net income (loss) per share - basic	$ (0.41)		$ (0.21)		$ 0.04	$ 0.97		$ 0.49		$ 0.19		$ 0.18	$ 0.18	$ 0.35	$ 1.03	$ 3.69
Net income (loss) per share - diluted	$ (0.41)		$ (0.21)		$ 0.04	$ 0.96		$ 0.48		$ 0.19		$ 0.18	$ 0.18	$ 0.35	$ 1.03	$ 3.63

[1] Included in net loss was income tax expense of $11.1 million from an increase in the valuation allowance on deferred tax assets.

[2] Included in loss from operations was expense of $10.1 million related to a legal settlement and mutual release, including legal fees incurred in the quarter.

[3] Included in net income was a benefit from income taxes of $16.6 million from the full recognition of certain deferred tax assets.

[4] Included in income from operations was severance expense of $6.3 million and gain on sale of member interest in Visa Europe Ltd. of $7.5 million.

[5] Included in income from operations was gain on sale of interest in Fabrix Systems Ltd. of $1.1 million.