Summary of Significant Accounting Policies - Schedule of Impact of Adoption on Consolidated Statement of Operations (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	$ 1,090	$ 963	$ 2,293	$ 2,008
Costs and expenses:
Cost of revenue	(559)	(488)	(1,185)	(1,057)
Operating income	122	83	269	173
Income before income taxes	115	18	251	75
Income tax expense	(48)	(19)	(98)	(71)
Net income (loss)	67	(1)	153	4
Less: Income attributable to noncontrolling interest		(2)		(3)
Net income (loss) attributable to Warner Music Group Corp.	67	$ (3)	153	$ 1
Balances Without Adoption Of ASC 606 | Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	1,080		2,263
Costs and expenses:
Cost of revenue	(545)		(1,169)
Operating income	126		255
Income before income taxes	119		237
Income tax expense	(50)		(92)
Net income (loss)	69		145
Less: Income attributable to noncontrolling interest	(1)		(1)
Net income (loss) attributable to Warner Music Group Corp.	68		144
Impact of Adoption | Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	10		30
Costs and expenses:
Cost of revenue	(14)		(16)
Operating income	(4)		14
Income before income taxes	(4)		14
Income tax expense	2		(6)
Net income (loss)	(2)		8
Less: Income attributable to noncontrolling interest	1		1
Net income (loss) attributable to Warner Music Group Corp.	$ (1)		$ 9