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Derivatives - Summary of Cash Flows are Expected to Occur (Detail) - EUR (€)
€ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of future net cash flow [Line Items]    
Cash inflows € 8,165 € 6,299
Cash outflows 4 5
Net cash flows 8,160 6,294
Not later than 1 year [member]    
Disclosure of future net cash flow [Line Items]    
Cash inflows 597 95
Net cash flows 597 95
1-5 years [member]    
Disclosure of future net cash flow [Line Items]    
Cash inflows 928 321
Cash outflows 1 1
Net cash flows 927 320
Over 5 through 10 years [member]    
Disclosure of future net cash flow [Line Items]    
Cash inflows 875 301
Cash outflows 3 3
Net cash flows 872 297
Over 10 years [member]    
Disclosure of future net cash flow [Line Items]    
Cash inflows 5,764 5,582
Net cash flows € 5,764 € 5,582