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Shareholders' equity - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
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    
Stock Option      
Shareholders Equity [Line Items]      
Weighted average antidilutive which were not included in the calculation of earnings per share (in shares) 600,000 600,000 3,400,000
Convertible Debt      
Shareholders Equity [Line Items]      
Debt instrument, face amount $ 400