Fair Value Measurements - Assets measured on a recurring basis - (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 41,973	$ 36,003
Investments, Fair Value Disclosure	116,066	139,137
Warrants to purchase equity securities	1,141	792
Total assets measured at fair value	$ 159,180	175,932
Fair value assumptions
Risk-free interest rate	1.80%
Expected term (in years)	4 years 4 months 24 days
Expected volatility	86.70%
Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 41,973	36,003
Investments, Fair Value Disclosure	116,066	131,533
Total assets measured at fair value	158,039	167,536
Level 2
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		7,604
Warrants to purchase equity securities	1,141	792
Total assets measured at fair value	1,141	8,396
U.S. Treasury notes
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	114,066	130,173
U.S. Treasury notes | Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	114,066	130,173
U. S. Government agency bonds
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		7,604
U. S. Government agency bonds | Level 2
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		7,604
Equity Securities
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	2,000	1,360
Equity Securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	$ 2,000	$ 1,360
Warrants
Fair value assumptions
Risk-free interest rate		1.80%
Expected term (in years)		4 years 8 months 12 days
Expected volatility		97.50%