Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Oct. 01, 2018	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ (1,061)		$ (1,272)
Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit		$ (1,133)
Reclassified from accounts receivable to other current liabilities		28
Impact of Adoption | Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ 160	$ 139