v3.10.0.1
Document entity information - shares
9 Months Ended
Jun. 30, 2018
Jul. 31, 2018
Entity Information [Abstract]    
Entity Registrant Name Viacom Inc.  
Entity Central Index Key 0001339947  
Current Fiscal Year End Date --09-30  
Entity Filer Category Large Accelerated Filer  
Document Type 10-Q  
Document Period End Date Jun. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Entity Well-known Seasoned Issuer Yes  
Entity Voluntary Filers No  
Entity Current Reporting Status Yes  
Common Class A    
Entity Common Stock, Shares Outstanding   49,430,905
Common Class B    
Entity Common Stock, Shares Outstanding   353,433,073