Income Taxes - Components of Income Taxes Provision (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Current provision (benefit):
Federal									$ 8,055	$ 2,321	$ 496
State									195	(366)	73
Total current provision									8,250	1,955	569
Deferred provision (benefit):
Federal									4,205	10,190	11,691
State									1,513	2,116	(52)
Total deferred provision									5,718	12,306	11,639
Total income tax provision	$ 3,386	$ 6,723	$ 1,610	$ 2,249	$ 1,852	$ 9,094	$ 1,484	$ 1,831	$ 13,968	$ 14,261	$ 12,208