Income Taxes - Current and Deferred Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Deferred Income Tax Assets And Liabilities
Income before income taxes	$ 117,316	$ 83,108	$ 170,004	$ 122,645
Current income tax expense	23,130	25,814	34,357	36,260
Deferred income tax expense (benefit)	9,022	(1,114)	12,623	222
Total income tax expense	$ 32,152	$ 24,700	$ 46,980	$ 36,482