Summary of significant accounting policies - Terms of contracts (Details) |
3 Months Ended |
|---|---|
Mar. 31, 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 |