Net Income Per Share - Reconciliation of the numerators and denominators of the basic and diluted net income per share computations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Net income (numerator):
Net income (in dollars)	$ 5,418	$ 4,890	$ 4,502	$ 5,757	$ 4,951	$ 4,797	$ 2,732	$ 4,973	$ 20,567	$ 17,453	$ 17,972
Shares (denominator)
Weighted average number of common shares outstanding used in basic computation									20,124	20,015	19,901
Common shares issuable upon the vesting of restricted stock									120	111	132
Common shares issuable upon conversion of preferred shares									10	10	10
Shares used in diluted computation									20,254	20,136	20,043
Net income per common share:
Basic (in dollars per share)									$ 1.02	$ 0.87	$ 0.90
Diluted (in dollars per share)	$ 0.27	$ 0.24	$ 0.22	$ 0.29	$ 0.25	$ 0.24	$ 0.14	$ 0.25	$ 1.02	$ 0.87	$ 0.90