Selected Quarterly Financial Data (unaudited) - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 51,313	$ 44,367	$ 39,631	$ 69,886	$ 67,205	$ 57,262	$ 44,049	$ 38,109	$ 205,197	$ 206,625	$ 102,814
Operating expenses	44,882	40,612	40,935	42,030	32,753	34,622	27,190	23,732	168,459	118,297	78,200
Other income (expense), net	2,274	2,415	2,245	1,885	1,429	1,338	1,066	960	8,819	4,793	2,333
Income tax benefit (expense)	486	866	3,204	(3,730)	(8,461)	(3,690)	(5,370)	(3,644)	826	(21,165)	(9,237)
Net income	$ 9,191	$ 7,036	$ 4,145	$ 26,011	$ 27,420	$ 20,288	$ 12,555	$ 11,693	$ 46,383	$ 71,956	$ 17,710
Net income per common share — basic	$ 0.47	$ 0.36	$ 0.21	$ 1.34	$ 1.41	$ 1.05	$ 0.65	$ 0.61	$ 2.37	$ 3.74	$ 0.93
Net income per common share — diluted	$ 0.44	$ 0.33	$ 0.20	$ 1.25	$ 1.30	$ 0.97	$ 0.61	$ 0.59	$ 2.21	$ 3.48	$ 0.91