Income Taxes - Current and Deferred Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Deferred Income Tax Assets And Liabilities
Income (loss) before income taxes	$ (2,051)	$ (33,153)	$ 120,594	$ 71,699
Current income tax expense (benefit)	(629)	(8,184)	35,631	31,363
Deferred income tax expense (benefit)	(1,376)	(1,871)	(1,154)	(11,206)
Total income tax expense (benefit)	$ (2,005)	$ (10,055)	$ 34,477	$ 20,157