Summary of Significant Accounting Policies - Summary of our comprehensive income (Details) - USD ($) $ 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
Accounting Policies [Abstract]
Net income	$ 5,418	$ 4,890	$ 4,502	$ 5,757	$ 4,951	$ 4,797	$ 2,732	$ 4,973	$ 20,567	$ 17,453	$ 17,972
Net change in unrealized gain (loss) on short term investments									54	(17)	(35)
Comprehensive income									$ 20,621	$ 17,436	$ 17,937