Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 22,543	$ 26,883	$ 22,822	$ 22,616	$ 30,816	$ 24,344	$ 22,676	$ 22,241	$ 94,864	$ 100,077	$ 81,336
Operating (loss) income	(1,993)	1,164	(156)	(266)	3,875	1,017	865	712	(1,251)	6,469	(8,799)
Net income (loss)	$ (2,946)	$ 2,459	$ 1,462	$ 148	$ 3,554	$ 1,466	$ 1,262	$ 1,310	$ 1,123	$ 7,592	$ (4,457)
Basic net income (loss) per share	$ (0.22)	$ 0.18	$ 0.11	$ 0.01	$ 0.27	$ 0.11	$ 0.09	$ 0.10	$ 0.08	$ 0.57	$ (0.34)
Diluted net income (loss) per share	$ (0.22)	$ 0.18	$ 0.11	$ 0.01	$ 0.26	$ 0.11	$ 0.09	$ 0.09	$ 0.08	$ 0.55	$ (0.34)