Cash and Cash Equivalents and Investments (Details 1) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents
Fair Value
Amortized Cost	$ 83,293	$ 46,600
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	83,293	46,600
U.S. Treasury Securities
Fair Value
Amortized Cost	90,533	227,911
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(57)	(107)
Estimated Fair Value	90,476	227,805
Corporate Debt Securities
Fair Value
Amortized Cost	23,054	72,188
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(6)	(90)
Estimated Fair Value	23,048	72,099
Total Short-term Investments
Fair Value
Amortized Cost	113,587	300,099
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(63)	(197)
Estimated Fair Value	113,524	299,904
Total Cash and Cash Equivalents and Investments
Fair Value
Amortized Cost	196,880	346,699
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(63)	(197)
Estimated Fair Value	$ 196,817	$ 346,504