Fair Value of Financial Instruments - Unrestricted Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Fair Value of Financial Instruments
Corporate debt securities, Amortized Cost	$ 94,049	$ 84,189	$ 67,058
Corporate debt securities, Gross Unrealized Gains	300	5	15
Corporate debt securities, Gross Unrealized Losses	(132)	(493)	(80)
Corporate debt securities, Fair Value	94,217	83,701	66,993
Contractual maturities of the unrestricted marketable securities held
Less Than 1 Year	25,427
1-5 years	68,790
Corporate debt securities, Fair Value	$ 94,217	$ 83,701	$ 66,993