Earnings per share - Securities excluded from the computation of diluted earnings per share (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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)	1,679,817	1,441,972	810,500
Number of share options and share-based awards unexercised (in shares)	3,904,614	3,950,794	5,157,292
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)	1,679,817	1,441,972	810,500