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Shareholders' equity - Additional Information (Detail) - $ / shares
 12 Months Ended
Apr. 04, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Shareholders Equity [Line Items]        
Common shares, authorized   200,000,000    
Common shares, par value (in dollars per share)   $ 1 $ 1  
Preference shares, authorized   500,000    
Per share market price of common stock (in dollars per share)   $ 61.32    
Strike price of warrants (in dollars per share)   $ 74.65    
Conversion of convertible notes, shares issued upon conversion 2,170,000 1,700,000 2,700,000 1,900,000
Convertible senior notes percentage   3.875% 3.875% 3.875%
Stock Option        
Shareholders Equity [Line Items]        
Weighted average antidilutive which were not included in the calculation of earnings per share (in shares)   3,400,000 5,600,000 6,300,000
Convertible Note        
Shareholders Equity [Line Items]        
Weighted average antidilutive which were not included in the calculation of earnings per share (in shares)   2,000,000 3,300,000 2,700,000