| Document and Entity Information | Oct. 16, 2019 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001339947 |
| Document Type | 8-K |
| Document Period End Date | Oct. 16, 2019 |
| Entity Registrant Name | VIACOM INC. |
| Entity Incorporation State Country Code | DE |
| Entity 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 | true |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Class A [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Class A common stock, par value $0.001 per share |
| Trading Symbol | VIA |
| Security Exchange Name | NASDAQ |
| Common Class B [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Class B common stock, par value $0.001 per share |
| Trading Symbol | VIAB |
| Security Exchange Name | NASDAQ |
