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Share-Based Payments - Schedule of Breakdown of the Closing Balance BSA 2 (Parenthetical) (Detail)
12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 15, 2017
EUR (€)
Dec. 05, 2017
EUR (€)
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Exercise price | €   € 38.18 € 39.00
Exercise price 65 [Member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Exercise price | $ $ 65