Summary of Significant Accounting Policies - Schedule of Impact of Adoption on Consolidated Statement of Operations (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	$ 1,203	$ 1,045
Costs and expenses:
Cost of revenue	(626)	(569)
Operating income	147	90
Income before income taxes	136	57
Income tax expense	(50)	(52)
Net income	86	5
Less: Income attributable to noncontrolling interest		(1)
Net income attributable to Warner Music Group Corp.	86	$ 4
Balances Without Adoption Of ASC 606 | Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	1,183
Costs and expenses:
Cost of revenue	(624)
Operating income	129
Income before income taxes	118
Income tax expense	(43)
Net income	75
Net income attributable to Warner Music Group Corp.	75
Impact of Adoption | Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	20
Costs and expenses:
Cost of revenue	(2)
Operating income	18
Income before income taxes	18
Income tax expense	(7)
Net income	11
Net income attributable to Warner Music Group Corp.	$ 11