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Financial instruments and concentration of credit risks - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value Measurements Disclosure [Line Items]    
Maturity period limit of foreign currency exchange instruments (in months) 3 months  
Receivables due from U.S. federal governmental agencies $ 75.8 $ 74.0
Foreign Exchange Forward Contract    
Fair Value Measurements Disclosure [Line Items]    
Notional amount of foreign exchange forward contracts not designated as hedging instruments $ 514.0 $ 428.9