Quarterly Financial Data (Unaudited) - Summary of Unaudited Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 18,154	$ 19,972	$ 16,699	$ 16,541	$ 17,364	$ 15,914	$ 14,415	$ 14,205	$ 71,366	$ 61,898	$ 57,439
Operating income	4,083	6,597	2,240	3,939	4,266	5,857	3,932	5,034	16,859	19,089	18,576
Net income	$ 2,577	$ 3,934	$ 821	$ 2,653	$ 1,358	$ 3,924	$ 3,051	$ 3,614	$ 9,985	$ 11,947	$ 12,031
Basic net income per share	$ 0.20	$ 0.31	$ 0.06	$ 0.20	$ 0.10	$ 0.30	$ 0.24	$ 0.27	$ 0.77	$ 0.92	$ 0.88
Diluted net income per share	$ 0.20	$ 0.30	$ 0.06	$ 0.20	$ 0.10	$ 0.30	$ 0.23	$ 0.27	$ 0.76	$ 0.90	$ 0.87