Net loss per share - Numerator and Denominator (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net loss	$ (33,738)	$ (35,167)	$ (80,270)	$ (115,314)
Denominator
Denominator for basic and diluted net loss per share (in shares)	41,296,740	34,088,741	38,433,749	34,002,952
Net loss per share:
Basic and diluted (in dollars per share)	$ (0.82)	$ (1.03)	$ (2.09)	$ (3.39)

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

No definition available.

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration