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Events After the Close of the Fiscal Year - Additional Information (Detail)
€ in Thousands
3 Months Ended 12 Months Ended
Feb. 14, 2018
Mar. 31, 2018
EUR (€)
Dec. 31, 2017
EUR (€)
Dec. 31, 2016
EUR (€)
Dec. 31, 2015
EUR (€)
Disclosure of non-adjusting events after reporting period [line items]          
Share-based compensation expense corresponding to the acceleration of the vesting period     € 30,781 € 34,353 € 10,419
Accelerated vesting period [member] | Scenario, Forecast [member]          
Disclosure of non-adjusting events after reporting period [line items]          
Share-based compensation expense corresponding to the acceleration of the vesting period   € 5,400      
Bottom of range [member] | Events after reporting period [member]          
Disclosure of non-adjusting events after reporting period [line items]          
Peanut allergy treatment covering age as per BLA 4        
Top of range [member] | Events after reporting period [member]          
Disclosure of non-adjusting events after reporting period [line items]          
Peanut allergy treatment covering age as per BLA 11