Shareholders' equity - Additional Information (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	500,000	1,700,000	2,700,000
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)	600,000	3,400,000	5,600,000
Convertible Note
Shareholders Equity [Line Items]
Weighted average antidilutive which were not included in the calculation of earnings per share (in shares)	300,000	2,000,000	3,300,000