Earnings per share - Securities excluded from the computation of diluted earnings per share (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 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)	701,600	163,900	894,876	4,416,300
Number of share options and share-based awards unexercised (in shares)	4,690,526	4,416,300	4,690,526	4,416,300
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)	701,600	163,900	894,876	2,913,300
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)	0	0	0	1,503,000