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, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Net income (numerator):
Net income (in dollars)	$ 7,503	$ 4,823	$ 4,899	$ 6,594	$ 5,418	$ 4,890	$ 4,502	$ 5,757	$ 23,819	$ 20,567	$ 17,453
Shares (denominator)
Weighted average number of common shares outstanding used in basic computation									20,232	20,124	20,015
Common shares issuable upon the vesting of restricted stock									136	120	111
Common shares issuable upon conversion of preferred shares									10	10	10
Shares used in diluted computation									20,378	20,254	20,136
Net income per common share:
Basic (in dollars per share)									$ 1.18	$ 1.02	$ 0.87
Diluted (in dollars per share)	$ 0.37	$ 0.24	$ 0.24	$ 0.32	$ 0.27	$ 0.24	$ 0.22	$ 0.29	$ 1.17	$ 1.02	$ 0.87