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Accounts receivable - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Receivables [Abstract]      
Revenue recognized in excess of billings on services contracts or unbilled accounts receivable $ 109.4 $ 98.0  
Unearned income deducted from accounts and notes receivable 12.5 7.0  
Allowance for doubtful accounts 22.0 22.8  
Provision expense for doubtful accounts reported in selling, general and administrative expenses $ 3.1 $ 2.2 $ 3.0