Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 23,038	$ 22,227	$ 19,058	$ 17,013	$ 20,058	$ 17,790	$ 17,503	$ 17,761	$ 81,336	$ 73,112	$ 71,366
Operating (loss) income	(2,441)	(6,250)	525	(633)	415	1,743	1,677	3,268	(8,799)	7,103	16,859
Net (loss) income	$ (1,753)	$ (2,682)	$ 1,534	$ (1,556)	$ 400	$ 720	$ 506	$ 2,300	$ (4,457)	$ 3,926	$ 9,985
Basic net (loss) income per share	$ (0.13)	$ (0.20)	$ 0.12	$ (0.12)	$ 0.03	$ 0.05	$ 0.04	$ 0.17	$ (0.34)	$ 0.30	$ 0.77
Diluted net (loss) income per share	$ (0.13)	$ (0.20)	$ 0.11	$ (0.12)	$ 0.03	$ 0.05	$ 0.04	$ 0.17	$ (0.34)	$ 0.29	$ 0.76