Financial Risk Management and Financial Instruments - Summary of Immediate Impact on Net Loss Before Tax (Details) € in Millions, £ in Millions, $ in Millions, $ in Millions |
12 Months Ended | |||||||
|---|---|---|---|---|---|---|---|---|
|
Dec. 31, 2018
EUR (€)
|
Dec. 31, 2018
USD ($)
|
Dec. 31, 2018
AUD ($)
|
Dec. 31, 2018
GBP (£)
|
Dec. 31, 2017
EUR (€)
|
Dec. 31, 2017
USD ($)
|
Dec. 31, 2017
AUD ($)
|
Dec. 31, 2017
GBP (£)
|
|
| Disclosure Of Financial Instruments [Abstract] | ||||||||
| (Increase)/decrease in loss before tax | € (3) | $ 74 | $ 7 | £ 3 | € 2 | $ 9 | $ 5 | £ (2) |