Fair Value Measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Assets measured at fair value on a recurring basis:
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
Subordinated convertible promissory notes
Assets measured at fair value on a recurring basis:
Amount invested in subordinated convertible promissory notes	6,100
Significant Unobservable Inputs Level 3 | Subordinated convertible promissory notes | Other Assets
Assets measured at fair value on a recurring basis:
Estimated fair value	5,800
Recurring basis
Assets measured at fair value on a recurring basis:
Total assets at fair value	69,783	$ 83,637
Recurring basis | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	13,198	13,928
Recurring basis | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	32,421	29,224
Recurring basis | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	14,869	22,359
Recurring basis | Corporate equity securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	9,295	18,126
Recurring basis | Actively Quoted Markets for Identical Assets Level 1
Assets measured at fair value on a recurring basis:
Total assets at fair value	13,198	13,928
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	13,198	13,928
Recurring basis | Significant Other Observable Inputs Level 2
Assets measured at fair value on a recurring basis:
Total assets at fair value	56,585	69,709
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	32,421	29,224
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	14,869	22,359
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	$ 18,126