Earnings per share - Securities excluded from the computation of diluted earnings per share (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of share options and share-based awards excluded from computation of earnings per share (in shares)	3,895,767	1,894,922	3,895,767	1,894,922
Number of share options and share-based awards unexercised (in shares)	3,895,767	4,197,980	3,895,767	4,197,980
Share options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of share options and share-based awards excluded from computation of earnings per share (in shares)	2,468,795	1,894,922	2,468,795	1,894,922
Share-based awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of share options and share-based awards excluded from computation of earnings per share (in shares)	1,426,972	0	1,426,972	0