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Share-based compensation - Outstanding options (Detail)
€ in Millions
Dec. 31, 2018
EUR (€)
Dec. 31, 2017
EUR (€)
Dec. 31, 2016
EUR (€)
Share-based compensation - Text Details (Detail) [Line Items]      
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 15.0 € 29.0 € 20.0
Euro [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 1,648,720    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 19.8    
Weighted average remaining contractual life of outstanding share options 2.3    
US Dollar [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 1,633,868    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 14.7    
Weighted average remaining contractual life of outstanding share options 2.3    
Price range five [Member] | US Dollar [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 368,670    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 0.7    
Weighted average remaining contractual life of outstanding share options 1.4    
Price range one [Member] | Euro [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 701,262    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 11.6    
Weighted average remaining contractual life of outstanding share options 2.7    
Price range one [Member] | US Dollar [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 595,675    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 3.3    
Weighted average remaining contractual life of outstanding share options 2.3    
Price range three [Member] | Euro [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 925,447    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 7.9    
Weighted average remaining contractual life of outstanding share options 2.0    
Price range three [Member] | US Dollar [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 23,925    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 0.3    
Weighted average remaining contractual life of outstanding share options 3.0    
Price range two [Member] | Euro [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 22,011    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 0.3    
Weighted average remaining contractual life of outstanding share options 3.0    
Price range two [Member] | US Dollar [Member]      
Share-based compensation - Text Details (Detail) [Line Items]      
Number of share options outstanding in share-based payment arrangement 645,598    
Intrinsic value of liabilities from share-based payment transactions for which counterparty's right to cash or other assets vested € 10.4    
Weighted average remaining contractual life of outstanding share options 2.7