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Subsequent events - Text Details (Detail) - EUR (€)
€ in Millions, shares in Millions
12 Months Ended 36 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2021
Jan. 29, 2019
Subsequent events - Text Details (Detail) [Line Items]          
Purchase of treasury shares € (514.0) € (318.0) € (589.0)    
Claim filed by LGE         € 64.6
Major ordinary share transactions [member]          
Subsequent events - Text Details (Detail) [Line Items]          
Purchase of treasury shares       € 1,500.0  
Shares acquired       46