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Earnings (loss) per share - Antidilutive securities excluded from the calculation of diluted EPS (Details) - shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Stock Options [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Securities excluded from calculation of diluted EPS 356,634 320,553 619,833
Service-Based Share Awards [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Securities excluded from calculation of diluted EPS 608,914 0 442,737