Summary of significant accounting policies - Revenue recognition (Details) |
12 Months Ended |
|---|---|
Dec. 31, 2017 | |
| 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 |