Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 20,058	$ 17,790	$ 17,503	$ 17,761	$ 18,154	$ 19,972	$ 16,699	$ 16,541	$ 73,112	$ 71,366	$ 61,898
Operating income	415	1,743	1,677	3,268	4,083	6,597	2,240	3,939	7,103	16,859	19,089
Net income	$ 400	$ 720	$ 506	$ 2,300	$ 2,577	$ 3,934	$ 821	$ 2,653	$ 3,926	$ 9,985	$ 11,947
Basic net income per share	$ 0.03	$ 0.05	$ 0.04	$ 0.17	$ 0.20	$ 0.31	$ 0.06	$ 0.20	$ 0.30	$ 0.77	$ 0.92
Diluted net income per share	$ 0.03	$ 0.05	$ 0.04	$ 0.17	$ 0.20	$ 0.30	$ 0.06	$ 0.20	$ 0.29	$ 0.76	$ 0.90