Summary of significant accounting policies - Revenue recognition - Terms of contracts (Details)	12 Months Ended
	Dec. 31, 2018 item
Minimum
Revenue recognition
Payment terms	30 days
Maximum
Revenue recognition
Payment terms	60 days
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
Military and retail
Revenue recognition
Cancellation period of renewal option prior end of current contract period	5 days
Multifamily
Revenue recognition
Number of performance obligations	2
Multifamily | Minimum
Revenue recognition
Initial term of the arrangement	3 years
Multifamily | Maximum
Revenue recognition
Initial term of the arrangement	5 years