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Income Tax - Additional Information (Detail) - EUR (€)
€ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Statements [Line Items]        
statutory tax rate 29.10% 29.10% 29.10%  
corporate income tax rate 21.00% 21.00% 21.00%  
Percentage on limitation on tax loss carryforwards after prior period 80.00%      
Percentage on limitation on tax loss carryforwards before prior period       100.00%
Effective income tax reconciliation permanent effect share listing expense € 153      
Indefinite Tax Losses Carryforward [Member]        
Statements [Line Items]        
Tax effect of tax losses 288 € 219    
Twenty Six Year Tax Losses Carryforward [Member]        
Statements [Line Items]        
Tax effect of tax losses € 26