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Share-Based Payments - Additional Information (Detail) - EUR (€)
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Exercise price of stock options € 34.07  
Number of ordinary shares issued 3,712,444  
Potential shares resulting from definitive acquistion of free shares 644,295  
IFRS 2 Share-based payments [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Share based payment expense € 8,000,000 € 16,800,000
BSA [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Description of warrants and stock options vesting period The warrants and stock options granted may be exercised at any time after a vesting period of between 0 and 4 years and become null and void after a period of 10 years from the date they are granted.  
Warrants and stock options expiration period 10 years  
BSA [member] | CEO and US [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Stock options vesting period 4 years  
Number of free shares granted 721,000  
Stock options forfeited period 10 years  
BSA [member] | One year [Member] | CEO and US [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Stock options vesting percentage 25.00%  
BSA [member] | Six months [member] | CEO and US [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Stock options vesting percentage 12.50%  
BSA [member] | Bottom of range [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Stock options vesting period 0 years  
BSA [member] | Bottom of range [member] | CEO and US [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Exercise price of stock options € 14.58  
BSA [member] | Top of range [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Stock options vesting period 4 years  
BSA [member] | Top of range [member] | CEO and US [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Exercise price of stock options € 16.99  
One free share plan [member] | French employees [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Number of free shares granted 100,000  
Free shares retention period 2 years