Income Taxes - Expiration Dates of Carryforwards (Parenthetical) (Detail)	12 Months Ended
	Sep. 30, 2017
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2025
Minimum [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2018
Maximum [Member]
Significant Tax Attributes And Dates Of Expiration [Line Items]
Expiration periods	2024