Financial instruments and concentration of credit risks - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Measurements Disclosure [Line Items]
Maturity period limit of foreign currency exchange instruments (in months)	3 months
Net fair value gain (loss) on foreign exchange forward contracts	$ 0.5	$ (4.4)
Receivables due from U.S. federal governmental agencies	74.0	99.0
Senior Notes due 2017
Fair Value Measurements Disclosure [Line Items]
Fair value of long-term debt	97.8	213.2
Convertible Senior Notes due 2021
Fair Value Measurements Disclosure [Line Items]
Fair value of long-term debt	379.8
Other income (expense), net
Fair Value Measurements Disclosure [Line Items]
Gain (loss) on foreign exchange forward contracts	(29.1)	15.6	$ 17.3
Prepaid Expenses and Other Current Assets
Fair Value Measurements Disclosure [Line Items]
Net fair value gain (loss) on foreign exchange forward contracts	2.4	2.2
Other Accrued Liabilities
Fair Value Measurements Disclosure [Line Items]
Net fair value gain (loss) on foreign exchange forward contracts	(1.9)	(6.6)
Foreign Exchange Contract
Fair Value Measurements Disclosure [Line Items]
Notional amount of foreign exchange forward contracts not designated as hedging instruments	$ 428.9	$ 940.1