Net income (loss) per share - Additional Information (Detail) - shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Series A Series B and Series C Preference Shares Convertible into Ordinary Shares [Member]
Earnings Per Share [Line Items]
Shares that anti-dilutive and excluded from the calculation of diluted net loss per share on weighted average basis	11,302,024	631,303,796

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