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Financial Risks - Summary of Unrealized Gains and Losses on Share Positions (Detail) - Share [member] - EUR (€)
€ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of unrealized gains and losses on share positions [line items]    
Cost basis € 290 € 371
Carrying value 312 391
Net unrealized gains / (losses) 23 (20)
Carrying value of securities with gross unrealized gains 254 304
Gross unrealized gains 47 40
Carrying value of securities with gross unrealized losses 58 86
Gross unrealized losses € (24) € (20)