Income Taxes - Expiration Dates of Carryforwards (Detail) $ in Millions	Sep. 30, 2018 USD ($)
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	$ 768
Credits	98
2019 to 2025 [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	282
Credits	15
2026 and thereafter [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	162
Credits	67
Indefinite Carry-Forwards [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Net operating loss carryforwards	324
Credits	$ 16