Income Taxes - Expiration Dates of Carryforwards (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	$ 798
Credits	212
2018 to 2024 [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	235
Credits	55
2025 and thereafter [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	226
Credits	136
Indefinite Carry-Forwards [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	337
Credits	$ 21