Related Parties (Details)	Sep. 30, 2020 trustee numberOfTrusts	Aug. 11, 2020
Related Party Transaction [Line Items]
SMR Trust ownership in NAI		80.00%
Number of continuing trusts | numberOfTrusts	2
Continuing trusts, voting stock ownership for each	40.00%
National Amusements, Inc. [Member]
Related Party Transaction [Line Items]
NAI ownership of CBS Corp. Class A Common Stock (percentage)	79.40%
NAI ownership of CBS Corp. Class A and Class B Common Stock on a combined basis (percentage)	10.20%
Number of trustees	7
Number of beneficiary trustees	2