v3.10.0.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2018
Oct. 31, 2018
Mar. 31, 2018
Entity Information [Line Items]      
Entity Registrant Name Viacom Inc.    
Entity Central Index Key 0001339947    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer Yes    
Entity Shell Company false    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Document Type 10-K    
Amendment Flag false    
Document Period End Date Sep. 30, 2018    
Common stock, Class A      
Entity Information [Line Items]      
Entity Common Stock, Shares Outstanding   49,430,905  
Entity Public Float     $ 395.6
Common stock, Class B      
Entity Information [Line Items]      
Entity Common Stock, Shares Outstanding   353,437,986  
Entity Public Float     $ 10,900.0