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Document And Entity Information - USD ($)
 12 Months Ended
Jul. 31, 2020
Oct. 09, 2020
Jan. 31, 2020
Document Information Line Items      
Entity Registrant Name IDT CORP    
Trading Symbol IDT    
Document Type 10-K    
Current Fiscal Year End Date --07-31    
Entity Public Float     $ 145,900,000
Amendment Flag false    
Entity Central Index Key 0001005731    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Accelerated Filer    
Entity Well-known Seasoned Issuer No    
Document Period End Date Jul. 31, 2020    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Small Business true    
Entity Emerging Growth Company false    
Entity Shell Company false    
Document Annual Report true    
Document Transition Report false    
Entity File Number 1-16371    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 22-3415036    
Entity Address, Address Line One 520 Broad Street    
Entity Address, City or Town Newark    
Entity Address, State or Province NJ    
Entity Address, Postal Zip Code 07102    
City Area Code (973)    
Local Phone Number 438-1000    
Title of 12(g) Security Class B common stock, par value $0.1 per share    
Security Exchange Name NYSE    
Entity Interactive Data Current Yes    
Class B common stock      
Document Information Line Items      
Entity Common Stock, Shares Outstanding   23,901,532  
Class A common stock      
Document Information Line Items      
Entity Common Stock, Shares Outstanding   1,574,326