Selected Quarterly Financial Data (Unaudited) - Summary of Selected Quarterly Financial Data (Detail) (USD $)	1 Months Ended	3 Months Ended								12 Months Ended
	Oct. 31, 2010	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2012	Jul. 31, 2011	Jul. 31, 2010
Quarterly Financial Data [Line Items]
REVENUES		$ 384,891,000	$ 379,719,000	$ 365,449,000	$ 376,777,000	$ 361,094,000	$ 340,188,000	$ 340,867,000	$ 309,767,000	$ 1,506,836,000	$ 1,351,916,000	$ 1,193,578,000
Direct cost of revenues		323,951,000	319,811,000	306,365,000	319,352,000	301,929,000	282,598,000	282,746,000	252,392,000	1,269,479,000	1,119,665,000	967,191,000
(Loss) income from operations		2,625,000	2,162,000	4,032,000	(11,062,000)	(818,000)	319,000	5,817,000	4,340,000	(2,243,000)	9,658,000	831,000
(Loss) income from continuing operations		37,480,000	3,449,000	2,929,000	(8,088,000)	(228,000)	5,158,000	4,323,000	13,173,000	35,770,000	22,426,000	6,572,000
Net (loss) income attributable to IDT Corporation		37,329,000	2,989,000	2,656,000	(4,326,000)	227,000	7,001,000	3,936,000	15,648,000	38,648,000	26,812,000	20,290,000
(Loss) income per share-basic, From continuing operations		$ 1.78	$ 0.14	$ 0.13	$ (0.40)	$ (0.19)	$ 0.25	$ 0.19	$ 0.63	$ 1.68	$ 1.05	$ 0.29
(Loss) income per share-basic, Net (loss) income		$ 1.78	$ 0.14	$ 0.13	$ (0.21)	$ 0.01	$ 0.34	$ 0.19	$ 0.76	$ 1.87	$ 1.30	$ 0.99
(Loss) income per share-diluted, From continuing operations		$ 1.69	$ 0.14	$ 0.12	$ (0.40)	$ (0.19)	$ 0.23	$ 0.18	$ 0.58	$ 1.57	$ 0.96	$ 0.28
(Loss) income per share-diluted, share Net (loss) income		$ 1.69	$ 0.14	$ 0.12	$ (0.21)	$ 0.01	$ 0.31	$ 0.18	$ 0.70	$ 1.75	$ 1.19	$ 0.94
Other operating gains (loss)			(1,100,000)	1,800,000	(11,300,000)			4,900,000	2,500,000	(10,540,000)	6,324,000	10,084,000
Income taxes benefit			2,300,000		3,300,000	2,300,000
Net litigation loss			6,500,000
Gain on sale of license										5,330,000
Reversal of a portion of the valuation allowance on deferred income tax assets		36,500,000
Gain on insurance claim								800,000	1,900,000
Gain (loss) on settlement of other claims									600,000		640,000	(1,500,000)
Other income from arbitration settlement	5,400,000								5,400,000
Benefit from reversal of income tax expense benefit							2,600,000		1,100,000
Gain on termination of cable telephony agreement								14,400,000			14,375,000
Patent infringement loss						1,100,000		9,800,000			10,828,000
Other loss								500,000
IDT Spectrum [Member]
Quarterly Financial Data [Line Items]
Gain on sale of license			$ 5,300,000
Number of license			8