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, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Accounting Policies [Abstract]
Net income	$ 4,526	$ 4,541	$ 4,150	$ 4,755	$ 4,602	$ 4,577	$ 4,034	$ 3,952	$ 17,972	$ 17,165	$ 16,659
Net change in unrealized gain (loss) on short term investments									(35)	(46)	75
Redemptions on long term investments											110
Comprehensive income									$ 17,937	$ 17,119	$ 16,844