Income Taxes - Expiration Dates of NOLs and Other Tax Credit Carryforwards Before Valuation Allowances (Detail) $ in Millions	Sep. 30, 2020 USD ($)
Significant Tax Attributes And Dates Of Expiration [Line Items]
NOLs	$ 653
Credits	125
2021 - 2027 [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
NOLs	168
Credits	50
2028 and thereafter [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
NOLs	198
Credits	73
Indefinite Carryforwards [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
NOLs	287
Credits	$ 2