Pension and Other Postretirement Benefits - Asset Allocations of Funded Pension Plan (Details)	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Percent Of Plan Assets Of Company Stock	1.00%	2.00%
Minimum \| Return seeking
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	70.00%
Minimum \| Liability hedging
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%
Minimum \| Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Maximum [Member] \| Return seeking
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	90.00%
Maximum [Member] \| Liability hedging
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	30.00%
Maximum [Member] \| Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%

Viacom Inc. 10-K