Financial Risk Management and Financial Instruments - Summary of Changes in Convertible Notes (Details) - EUR (€) € in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Disclosure Of Financial Instruments [Abstract] | ||
| Beginning balance | € 944 | € 1,106 |
| Changes in fair value | 221 | 666 |
| Effect of changes in foreign exchange rates | (20) | (142) |
| Issuance of shares upon exchange of Convertible Notes | (1,145) | (686) |
| Ending balance | € 0 | € 944 |