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Summary of significant accounting policies - Revenue recognition (Details)
 6 Months Ended
Jun. 30, 2017
DAS license agreements | Minimum  
Revenue recognition  
Term of the arrangement used to determine revenue recognition 5 years
DAS license agreements | Maximum  
Revenue recognition  
Term of the arrangement used to determine revenue recognition 20 years
Wholesale-Wi-Fi | Minimum  
Revenue recognition  
Term of the arrangement used to determine revenue recognition 1 year
Wholesale-Wi-Fi | Maximum  
Revenue recognition  
Term of the arrangement used to determine revenue recognition 5 years