Fair Value Measurements (Details 1) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Level 1
Fair Value
Cash and Cash Equivalents	$ 2,455	$ 45,093
Short-term Investments	191,248	227,805
Total Cash and Cash Equivalents and Investments	193,703	272,898
Accrued Liabilities		0
Total Liabilities		0
Fair Value, Level 2
Fair Value
Cash and Cash Equivalents	0	1,507
Short-term Investments	66,040	72,099
Total Cash and Cash Equivalents and Investments	66,040	73,606
Accrued Liabilities		0
Total Liabilities		0
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
Accrued Liabilities		18,912
Total Liabilities		18,912
Fair Value, Total
Fair Value
Cash and Cash Equivalents	2,455	46,600
Short-term Investments	257,288	299,904
Total Cash and Cash Equivalents and Investments	$ 259,743	346,504
Accrued Liabilities		18,912
Total Liabilities		$ 18,912