Summary of Significant Accounting Policies - Summary of our comprehensive income (Details) (USD $) In Thousands, 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
Accounting Policies [Abstract]
Net income	$ 4,951	$ 4,797	$ 2,732	$ 4,973	$ 4,526	$ 4,541	$ 4,150	$ 4,755	$ 17,453	$ 17,972	$ 17,165
Net change in unrealized loss on short term investments									(17)	(35)	(46)
Comprehensive income									$ 17,436	$ 17,937	$ 17,119