| Related party (Narrative) (Details) - $ / shares | Jan. 31, 2019 | Jul. 19, 2018 | Apr. 30, 2018 | 
|---|---|---|---|
| Related Party Transaction [Line Items] | |||
| Per share price | $ 0.001 | $ 0.001 | |
| Common Stock [Member] | |||
| Related Party Transaction [Line Items] | |||
| Treasury stock repurchased | 1,000,000 | ||
| Per share price | $ 7.40 | ||
| Sale of common stock per share price | $ 7.40 | 
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- Definition Face amount or stated value per share of common stock. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- 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. 
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- Definition Per share or per unit amount of equity securities issued. No definition available. 
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- 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 
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- Details 
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