Earnings per share - Securities excluded from the computation of diluted earnings per share (Details) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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)	3,837,579	4,196,412
Number of share options and share-based awards unexercised (in shares)	3,837,579	4,196,412
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)	2,415,014	2,754,007
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)	1,422,565	1,442,405