Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016	Mar. 08, 2017
Privately Held Companies
Investment fair value
Change in fair value of preferred stock	$ 0
Subordinated convertible promissory notes | Privately Held Companies
Investment fair value
Amount invested in subordinated convertible promissory notes		$ 1,400	$ 6,100
Subordinated convertible promissory notes | Privately Held Companies | Other Assets
Investment fair value
Estimated fair value				$ 7,100
Significant Unobservable Inputs Level 3 | Subordinated convertible promissory notes | Other Assets
Investment fair value
Estimated fair value			5,800
Recurring basis
Assets measured at fair value on a recurring basis:
Total assets at fair value	85,619	85,619	69,783
Recurring basis | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	9,677	9,677	13,198
Recurring basis | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	39,632	39,632	32,421
Recurring basis | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	36,310	36,310	14,869
Recurring basis | Corporate equity securities
Assets measured at fair value on a recurring basis:
Total assets at fair value			9,295
Recurring basis | Actively Quoted Markets for Identical Assets Level 1
Assets measured at fair value on a recurring basis:
Total assets at fair value	9,677	9,677	13,198
Recurring basis | 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	9,677	9,677	13,198
Recurring basis | Significant Other Observable Inputs Level 2
Assets measured at fair value on a recurring basis:
Total assets at fair value	75,942	75,942	56,585
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	39,632	39,632	32,421
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	$ 36,310	$ 36,310	14,869
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate equity securities
Assets measured at fair value on a recurring basis:
Total assets at fair value			$ 9,295