Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Oct. 01, 2018	Sep. 30, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accumulated deficit	$ (1,043)		$ (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
Difference between Revenue Guidance in Effect before and after Topic 606 | Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accumulated deficit	$ 147	$ 139