Fair Value Measurements (Details 1) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value
Cash and Cash Equivalents	$ 20,421	$ 61,661
Short-term Investments	241,851	249,127
Total Cash and Cash Equivalents and Investments	262,272	310,788
Fair Value, Level 1
Fair Value
Cash and Cash Equivalents	20,421	61,661
Short-term Investments	213,985	222,148
Total Cash and Cash Equivalents and Investments	234,406	283,809
Fair Value, Level 2
Fair Value
Cash and Cash Equivalents	0	0
Short-term Investments	27,866	26,979
Total Cash and Cash Equivalents and Investments	27,866	26,979
Fair Value, Level 3
Fair Value
Cash and Cash Equivalents	0	0
Short-term Investments	0	0
Total Cash and Cash Equivalents and Investments	$ 0	$ 0