Summary of Significant Accounting Policies - Summary of our comprehensive income (Details) - USD ($) $ 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
Accounting Policies [Abstract]
Net income	$ 7,503	$ 4,823	$ 4,899	$ 6,594	$ 5,418	$ 4,890	$ 4,502	$ 5,757	$ 23,819	$ 20,567	$ 17,453
Net change in unrealized gain (loss) on short term investments										54	(17)
Comprehensive income									$ 23,819	$ 20,621	$ 17,436