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Accounts receivable - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Receivables [Abstract]      
Revenue recognized in excess of billings on services contracts or unbilled accounts receivable $ 94.4 $ 109.4  
Unearned income deducted from accounts receivable 8.4 12.5  
Allowance for doubtful accounts 13.7 22.0  
Provision for doubtful accounts reported in selling, general and administrative expenses, (income) expense $ (5.1) $ 3.1 $ 2.2