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Goodwill and Intangible Assets - Additional Information (Detail)
12 Months Ended
Dec. 31, 2017
EUR (€)
Product_Category
GeographicalAreas
Dec. 31, 2016
EUR (€)
Disclosure of detailed information about intangible assets [Line Items]    
Number of product categories included in cash generating units | Product_Category 4  
Number of geographical areas included in cash generating units | GeographicalAreas 3  
Goodwill not allocated to cash generating units € 2,405,000,000  
Goodwill € 16,881,000,000 € 17,624,000,000
Pre-tax discount rate 7.40% 7.40%
Projection period 5 years  
Spreads business [member]    
Disclosure of detailed information about intangible assets [Line Items]    
Goodwill € 2,228,000,000  
Indefinite-life intangible assets 82,000,000  
Goodwill [member]    
Disclosure of detailed information about intangible assets [Line Items]    
Impairment charges 0  
Indefinite-life intangible assets [member]    
Disclosure of detailed information about intangible assets [Line Items]    
Impairment charges € 0  
Top of range [member]    
Disclosure of detailed information about intangible assets [Line Items]    
Amortisation period of finite-life intangible assets Ten years