Document entity information - shares
6 Months Ended
Mar. 31, 2019
Apr. 30, 2019
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, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Current Reporting Status - Indicate 'Yes' or 'No' whether registrants (1) have filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that registrants were required to file such reports), and (2) have been subject to such filing requirements for the past 90 days. This information should be based on the registrant's current or most recent filing containing the related disclosure. Yes  
Common Class A    
Entity Common Stock, Shares Outstanding   49,430,635
Common Class B    
Entity Common Stock, Shares Outstanding   353,644,397