Earnings per share - Securities excluded from the computation of diluted earnings per share (Details) - shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share
Number of share options and share-based awards excluded from computation of earnings per share (in shares)	3,976,445	1,679,817	1,441,972
Number of share options and share-based awards unexercised (in shares)	3,976,445	3,904,614	3,950,794
Share options
Antidilutive Securities Excluded from Computation of Earnings Per Share
Number of share options and share-based awards excluded from computation of earnings per share (in shares)	2,704,707	1,679,817	1,441,972
Share-based awards
Antidilutive Securities Excluded from Computation of Earnings Per Share
Number of share options and share-based awards excluded from computation of earnings per share (in shares)	1,271,738	0	0

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://www.xbrl.org/2003/role/presentationRef
-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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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