XML 177 R154.htm IDEA: XBRL DOCUMENT v3.19.1
Summary of significant accounting policies - Narrative (Details) - USD ($)
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Disclosure of detailed information about intangible assets [line items]      
Increase (decrease) retained earnings   $ 0  
IAS 16      
Disclosure of detailed information about intangible assets [line items]      
Increase (decrease) in property, plant and equipment $ 545,000,000    
Increase (decrease) in deferred tax liability (asset) $ 139,000,000    
IAS 8      
Disclosure of detailed information about intangible assets [line items]      
Increase (decrease) in deferred tax liability (asset)     $ 12,000,000
Increase (decrease) retained earnings     45,000,000
Increase (decrease) in investment property     $ 40,000,000
Bottom of range | Trademarks      
Disclosure of detailed information about intangible assets [line items]      
Useful lives of intangible assets   10 years  
Bottom of range | Software      
Disclosure of detailed information about intangible assets [line items]      
Useful lives of intangible assets   3 years  
Top of range | Trademarks      
Disclosure of detailed information about intangible assets [line items]      
Useful lives of intangible assets   20 years  
Top of range | Software      
Disclosure of detailed information about intangible assets [line items]      
Useful lives of intangible assets   5 years  
Expiration date 2018      
Disclosure of detailed information about intangible assets [line items]      
Power agreement term (in years)   10 years  
Expiration date 2024      
Disclosure of detailed information about intangible assets [line items]      
Power agreement term (in years)   15 years  
Expiration date 2025      
Disclosure of detailed information about intangible assets [line items]      
Power agreement term (in years)   15 years