Fair value measurements - Assets measured on a recurring basis - (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair value assumptions
Risk-free interest rate	2.40%	2.00%
Expected term (in years)	3 years 4 months 24 days	3 years 8 months 12 days
Expected volatility	104.30%	103.50%
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Investments	$ 79,147	$ 138,622
Total assets measured at fair value	219,118	169,660
Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Investments	79,147	138,622
Total assets measured at fair value	218,416	169,091
Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	702	569
U.S. Treasury notes | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Investments	77,827	137,522
U.S. Treasury notes | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Investments	77,827	137,522
Equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Investments	1,320	1,100
Equity securities | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Investments	1,320	1,100
Warrants to purchase equity securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	702	569
Warrants to purchase equity securities | Level 2 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Warrants to purchase equity securities	702	569
Money market funds | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	139,269	30,469
Money market funds | Level 1 | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 139,269	$ 30,469