| Cover Page | Nov. 14, 2019 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 14, 2019 |
| Entity Registrant Name | VIACOM INC. |
| Entity Incorporation, State or Country Code | DE |
| Contained File Information, File Number | 001-32686 |
| Entity Tax Identification Number | 20-3515052 |
| Entity Address, Address Line One | 1515 Broadway, |
| Entity Address, City or Town | New York, |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10036 |
| City Area Code | 212 |
| Local Phone Number | 258-6000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001339947 |
| Amendment Flag | false |
| Common Class A | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Class A common stock, par value $0.001 per share |
| Trading Symbol | VIA |
| Security Exchange Name | NASDAQ |
| Common Class B | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Class B common stock, par value $0.001 per share |
| Trading Symbol | VIAB |
| Security Exchange Name | NASDAQ |
