Net loss per share - Antidilutive (Details) - shares	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Net loss per share
Total shares excluded from calculation (in shares)	8,750,584	7,548,038
Stock Options
Net loss per share
Total shares excluded from calculation (in shares)	8,166,403	7,124,052
Unvested restricted stock awards and units
Net loss per share
Total shares excluded from calculation (in shares)	584,181	423,986

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration