Net Income Per Share - Reconciliation of the numerators and denominators of the basic and diluted net income per share computations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Net income (numerator):
Net income (in dollars)	$ 4,951	$ 4,797	$ 2,732	$ 4,973	$ 4,526	$ 4,541	$ 4,150	$ 4,755	$ 17,453	$ 17,972	$ 17,165
Shares (denominator)
Weighted average number of common shares outstanding used in basic computation									20,015	19,901	19,926
Common shares issuable upon the vesting of restricted stock									111	132	113
Common shares issuable upon conversion of preferred shares									10	10	10
Shares used in diluted computation									20,136	20,043	20,049
Net income per common share:
Basic (in dollars per share)									$ 0.87	$ 0.90	$ 0.86
Diluted (in dollars per share)	$ 0.25	$ 0.24	$ 0.14	$ 0.25	$ 0.23	$ 0.23	$ 0.21	$ 0.24	$ 0.87	$ 0.90	$ 0.86