Share-based Payment Expense - Summary of Movement and Options Outstanding Under Share Option Schemes (Parenthetical) (Detail) - Share Option Schemes Excluding Savings Related Share Option Schemes [member]	12 Months Ended
	Dec. 31, 2017 EUR (€) yr	Dec. 31, 2016 EUR (€)	Dec. 31, 2015 EUR (€)
Disclosure Of Share Based Compensation Expense [Line Items]
Weighted average share price | €	€ 32.24	€ 29.70	€ 25.51
Description of option Life share options granted | yr	10