Income Taxes - Expiration Dates of NOLs and Other Tax Credit Carryforwards Before Valuation Allowances (Parenthetical) (Detail)	12 Months Ended
	Sep. 30, 2020
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2028
Minimum [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2021
Maximum [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2027