Short-Term Investments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) instrument	Dec. 31, 2017 USD ($)
Short-Term Investments
Amortized Cost	$ 22,280
Gross Unrealized Losses	(4)
Fair Value	$ 22,276	$ 0
Number of short term investments in unrealized loss position | instrument	7
Number of short term investments in an unrealized loss position more than 12 months | instrument	0
Aggregate fair value of the investments in an unrealized loss position	$ 7,300
Corporate debt securities
Short-Term Investments
Amortized Cost	7,280
Gross Unrealized Losses	(4)
Fair Value	7,276
Reverse repurchase agreements
Short-Term Investments
Amortized Cost	15,000
Fair Value	$ 15,000