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Earnings per common share - Additional Information (Detail) - shares
shares in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Convertible preferred stock      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive securities (in shares)     1,171
Stock options and restricted stock units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive securities (in shares) 3,553 2,915 1,929
5.50% Convertible Senior Notes due 2021      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive securities (in shares) 17,230    
Convertible senior notes interest rate 5.50%