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Financial Risks - Summary of Composition of Impairment Losses and Recoveries (Parenthetical) (Detail) - EUR (€)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of impairment loss recognised or reversed [line items]    
Net impairments and (recoveries) € (17,000,000) € (10,000,000)
Bottom of range [member]    
Disclosure of impairment loss recognised or reversed [line items]    
Net impairments and (recoveries) € 25,000,000 € 25,000,000