Summary of significant accounting policies - Income taxes (Details) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Summary of significant accounting policies | ||
| Deferred Tax Assets, Net | $ 0.0 | |
| Uncertain tax positions | 0.0 | $ 0.0 |
| Interest or penalties recorded | $ 0.0 | $ 0.0 |