Summary of significant accounting policies - Non controlling interests (Details) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Non-controlling interests | |||
| Distributions to non-controlling interest holders | $ 125 | $ 286 | $ 500 |
| Chicago Concourse Development Group, LLC | |||
| Non-controlling interests | |||
| Percentage of net profits less capital expenditures of the preceding year allocated to non-controlling interest holders | 30.00% | ||
| Distributions to non-controlling interest holders | $ 125 | 286 | 500 |
| Boingo Holding Participacoes Ltda. | |||
| Non-controlling interests | |||
| Distributions to non-controlling interest holders | $ 0 | $ 0 | $ 0 |