Marketable Securities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Cash equivalents
Carrying Value	$ 6,294	$ 22,858
Short-term investments
Amortized Cost	91,694	97,093
Unrealized Gains	86	17
Unrealized Losses	0	(7)
Aggregate Fair Value	91,780	97,103
Long-term investments
Amortized Cost	5,254
Unrealized Gains	8
Unrealized Losses	0
Aggregate Fair Value	5,262	0
Total marketable securities, Amortized Cost	103,242	119,951
Total marketable securities, Unrealized Gains	94	17
Total marketable securities, Unrealized Losses	0	(7)
Total marketable securities, Aggregate Fair Value	103,336	119,961
U.S. treasury securities
Cash equivalents
Carrying Value		1,000
Short-term investments
Amortized Cost	77,526	65,491
Unrealized Gains	65	2
Unrealized Losses	0	(4)
Aggregate Fair Value	77,591	65,489
Corporate bonds
Short-term investments
Amortized Cost	14,168	19,708
Unrealized Gains	21	15
Unrealized Losses	0	(3)
Aggregate Fair Value	14,189	19,720
Long-term investments
Amortized Cost	5,254
Unrealized Gains	8
Unrealized Losses	0
Aggregate Fair Value	5,262
Commercial paper
Cash equivalents
Carrying Value		6,635
Short-term investments
Amortized Cost		11,894
Unrealized Gains		0
Unrealized Losses		0
Aggregate Fair Value		11,894
Money market funds
Cash equivalents
Carrying Value	$ 6,294	$ 15,223
Minimum
Marketable Securities [Line Items]
Long-term investments, maturity (in months)	15 months
Maximum
Marketable Securities [Line Items]
Long-term investments, maturity (in months)	17 months