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Shareholders' equity - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2007
Shareholders Equity [Line Items]        
Common shares, authorized 200,000,000      
Common shares, par value $ 1 $ 1    
Preference shares, authorized 500,000      
Per share market price of common stock $ 61.32      
Strike price of warrants $ 74.65      
Conversion of convertible notes, shares issued upon conversion 2,700,000 1,900,000 600,000  
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 5,600,000 6,300,000 7,700,000  
Convertible Note        
Shareholders Equity [Line Items]        
Weighted average antidilutive which were not included in the calculation of earnings per share 3,300,000 2,700,000 1,600,000  
Common Stock        
Shareholders Equity [Line Items]        
Repurchase of stock, authorized amount       $ 300,000,000