v3.8.0.1
Document entity information - shares
6 Months Ended
Mar. 31, 2018
Apr. 13, 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 Mar. 31, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q2  
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   49,431,181
Common Class B    
Entity Common Stock, Shares Outstanding   352,973,760