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Events After the Reporting Period - Restructuring (Details) - EUR (€)
€ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of non-adjusting events after reporting period [line items]        
Restructuring expenses   € 19 € 182 € 28
Restructuring | Lowest        
Disclosure of non-adjusting events after reporting period [line items]        
Restructuring expenses € 800      
Restructuring | Highest        
Disclosure of non-adjusting events after reporting period [line items]        
Restructuring expenses € 950