Fair Value Measurements - Assets measured on a recurring basis - (Details) - USD ($) $ in Thousands		9 Months Ended
	Sep. 02, 2016	Sep. 30, 2016	Dec. 31, 2015
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		$ 130,062	$ 193,036
Warrants to purchase equity securities		1,559
Total assets measured at fair value		194,629	222,637
Fair value assumptions
Risk-free interest rate	1.20%
Expected term (in years)	5 years
Expected volatility	92.30%
Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		114,902	157,981
Total assets measured at fair value		177,910	187,582
Level 2
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		15,160	35,055
Warrants to purchase equity securities		1,559
Total assets measured at fair value		16,719	35,055
U.S. Treasury notes
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		112,486	157,981
U.S. Treasury notes | Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		112,486	157,981
U. S. Government agency bonds
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		15,160	35,055
U. S. Government agency bonds | Level 2
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		15,160	35,055
Equity securities
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		2,416
Equity securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		$ 2,416
Warrants
Fair value assumptions
Risk-free interest rate		1.10%
Expected term (in years)		4 years 10 months 24 days
Expected volatility		92.30%
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		$ 63,008	29,601
Money market funds | Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents		$ 63,008	$ 29,601