| Related party (Narrative) (Details) - $ / shares | Apr. 30, 2019 | Jul. 19, 2018 | Apr. 30, 2018 | Apr. 23, 2018 | 
|---|---|---|---|---|
| Related Party Transaction [Line Items] | Ìý | Ìý | Ìý | Ìý | 
| Per share price | $ 0.001 | Ìý | $ 0.001 | Ìý | 
| Sale of common stock per share price | Ìý | Ìý | Ìý | $ 7.15 | 
| Common Stock [Member] | Ìý | Ìý | Ìý | Ìý | 
| Related Party Transaction [Line Items] | Ìý | Ìý | Ìý | Ìý | 
| Treasury stock repurchased | Ìý | 1,000,000 | Ìý | Ìý | 
| Per share price | Ìý | $ 7.40 | Ìý | Ìý | 
| Sale of common stock per share price | Ìý | $ 7.40 | Ìý | Ìý | 
| X | ||||||||||
| 
- Definition Face amount or stated value per share of common stock. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
 | 
| X | ||||||||||
| 
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
 | 
| X | ||||||||||
| 
- Definition Per share or per unit amount of equity securities issued. No definition available. 
 | 
| X | ||||||||||
| 
- Definition Number of previously issued common shares repurchased by the issuing entity and held in treasury. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
 | 
| X | ||||||||||
| 
- Details 
 |