XML 176 R153.htm IDEA: XBRL DOCUMENT v3.8.0.1
Summary of significant accounting policies - Narrative (Details)
 12 Months Ended
Dec. 31, 2017
contract
Disclosure of detailed information about intangible assets [line items]  
Number of power agreements 2
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