Shareholders' equity - Additional Information (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shareholders Equity [Line Items]
Common shares, authorized (in shares)	200,000,000
Common shares, par value (in dollars per share)	$ 1	$ 1
Preference shares, authorized (in shares)	500,000
Stock Option
Shareholders Equity [Line Items]
Weighted average antidilutive which were not included in the calculation of earnings per share (in shares)	100,000	100,000	600,000