Assets - Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash equivalents
Total	$ 245,525	$ 279,736
Short-term investments
Total	65,130	47,780
Transfer of assets from Level 1 to Level 2	0
Transfer of assets from Level 2 to Level 1	0
Transfer of Liabilities from Level 1 to Level 2	0
Transfer of Liabilities from Level 2 to Level 1	0
Recurring
Cash equivalents
Total	6,496	12,490
Short-term investments
Total	65,130	47,780
Recurring | U.S. treasuries
Cash equivalents
Total	6,496	12,490
Short-term investments
Total	42,228	33,895
Recurring | Corporate debt
Short-term investments
Total	10,522	10,886
Recurring | Commercial paper
Short-term investments
Total	12,380	2,999
Recurring | Level 1
Cash equivalents
Total	6,496	12,490
Short-term investments
Total	42,228	33,895
Recurring | Level 1 | U.S. treasuries
Cash equivalents
Total	6,496	12,490
Short-term investments
Total	42,228	33,895
Recurring | Level 2
Short-term investments
Total	22,902	13,885
Recurring | Level 2 | Corporate debt
Short-term investments
Total	10,522	10,886
Recurring | Level 2 | Commercial paper
Short-term investments
Total	$ 12,380	$ 2,999