Earnings per common share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Basic earnings per share
Net earnings	$ 100.4	$ 105.6
Weighted average number of common shares outstanding	41,140,321	36,306,880
Deferred share units outstanding	68,007	70,476
Weighted average number of common shares outstanding - basic	41,208,328	36,377,356
Earnings per common share - basic	$ 2.44	$ 2.90
Diluted earnings per share
Net earnings - basic	$ 100.4	$ 105.6
Effect of potentially dilutive share appreciation rights		(2.4)
Net earnings - dilutive	$ 100.4	$ 103.2
Weighted average number of common shares outstanding - basic	41,208,328	36,377,356
Effect of potentially dilutive share appreciation rights	24,743	140,136
Effect of restricted share units	76,810
Weighted average number of common shares outstanding - diluted	41,309,881	36,517,492
Earnings per common share - diluted	$ 2.43	$ 2.83
Share appreciation rights excluded from diluted weighted average number of shares	4,501,000	3,665,000

The weighted average number of ordinary shares outstanding plus the weighted average number of ordinary shares that would be issued on the conversion of all the dilutive potential ordinary shares into ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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