Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets measured at fair value on a recurring basis:
Total assets at fair value	$ 101,205	$ 95,601
Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	11,501	10,215
Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	34,945	30,272
Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	49,566	50,260
Corporate equity securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	3,791	3,525
Convertible promissory notes
Assets measured at fair value on a recurring basis:
Total assets at fair value	1,402	1,329
Actively Quoted Markets for Identical Assets Level 1
Assets measured at fair value on a recurring basis:
Total assets at fair value	11,501	10,215
Actively Quoted Markets for Identical Assets Level 1 | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	11,501	10,215
Significant Other Observable Inputs Level 2
Assets measured at fair value on a recurring basis:
Total assets at fair value	88,302	84,057
Significant Other Observable Inputs Level 2 | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	34,945	30,272
Significant Other Observable Inputs Level 2 | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	49,566	50,260
Significant Other Observable Inputs Level 2 | Corporate equity securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	3,791	3,525
Significant Unobservable Inputs Level 3
Assets measured at fair value on a recurring basis:
Total assets at fair value	1,402	1,329
Significant Unobservable Inputs Level 3 | Convertible promissory notes
Assets measured at fair value on a recurring basis:
Total assets at fair value	$ 1,402	$ 1,329