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Foreign currency - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Intercompany Foreign Currency Balance [Line Items]      
Gain (loss) recognized on foreign exchange $ (9.9) $ 2.3 $ 8.1
Facility Closing      
Intercompany Foreign Currency Balance [Line Items]      
Gain (loss) recognized on foreign exchange $ (11.8)