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Impairment Charges / (Reversals) - Summary of Impairment Reversals on Financial Assets, Excluding Receivables (Detail) - EUR (€)
€ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment reversals on financial assets, excludes receivables € (73) € (39) € (32)
Debt securities and money market instruments [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment reversals on financial assets, excludes receivables (67) (34) (17)
Loans [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment reversals on financial assets, excludes receivables (5) (3) (13)
Other [member]      
Disclosure of impairment loss and reversal of impairment loss [line items]      
Impairment reversals on financial assets, excludes receivables € (1) € (2) € (2)