Summary of significant accounting policies - Terms of contracts (Details)	6 Months Ended
	Jun. 30, 2018
DAS license agreements | Minimum
Revenue recognition
Initial term of the arrangement	5 years
DAS license agreements | Maximum
Revenue recognition
Initial term of the arrangement	20 years
Wholesale-Wi-Fi | Minimum
Revenue recognition
Initial term of the arrangement	1 year
Wholesale-Wi-Fi | Maximum
Revenue recognition
Initial term of the arrangement	3 years