Income Taxes - Expiration Dates of Carryforwards (Detail) $ in Millions	Sep. 30, 2019 USD ($)
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	$ 711
Credits	99
2020 - 2026 [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	262
Credits	40
2027 and thereafter [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	168
Credits	57
Indefinite Carryforwards [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	281
Credits	$ 2