Financial Instruments - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 EUR (€) DerivativeContract	Dec. 31, 2017 DerivativeContract
Senior Notes
Financial Instrument At Fair Value [Line Items]
Debt instrument base principal amount	€ 1,000
Debt instrument estimated market price per €1000 principal amount	€ 969
Currency Forward Contracts
Financial Instrument At Fair Value [Line Items]
Number of derivative contacts outstanding | DerivativeContract	0	0