Discontinued Operations - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended			1 Months Ended				12 Months Ended
	Oct. 28, 2011 Genie Energy [Member]	Oct. 28, 2011 IDT Energy [Member]	Oct. 28, 2011 Genie Oil and Gas [Member]	Sep. 30, 2011 IDT Entertainment [Member]	Aug. 30, 2011 IDT Entertainment [Member]	Jul. 31, 2013 Straight Path Spectrum, Inc. [Member]	Jul. 31, 2013 Straight Path IP Group, Inc. [Member]	Jul. 31, 2013 Straight Path Communications, Inc. [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership percentage at the time of spin-off	99.30%	100.00%	0.92%			100.00%	84.50%
Description of shares issued in spin-off transaction	Each of the Company's stockholders received one share of Genie Class A common stock for every share of the Company's Class A common stock and one share of Genie Class B common stock for every share of the Company's Class B common stock held of record as of the close of business on October 21, 2011.							Each of the Company's stockholders received one share of Straight Path Class A common stock for every two shares of the Company's Class A common stock and one share of Straight Path Class B common stock for every two shares of the Company's Class B common stock held of record date as of the close of business on July 25, 2013
Discontinued operations amount funded to spin-off	$ 106.0							$ 15.0
Payment to Liberty Media in connection with IDT Entertainment					3.5
Period to receive additional consideration from Liberty Media Corporation					5 years
Gain (loss) recorded from prior period adjustments				$ 2.0