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Income taxes - Additional Information (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2018
Apr. 30, 2017
Oct. 31, 2018
Oct. 31, 2017
Income Taxes [Line Items]            
Provision for income tax as percentage of income (loss) before income taxes 20.00% 47.00% 20.00% 26.00%    
Unrecognized tax benefits $ 0   $ 0     $ 0
Accrued interest and penalties related to uncertain tax positions $ 0   0     $ 0
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent           35.00%
Increase (Decrease) in Deferred Liabilities     41,000      
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount     41,000      
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount     $ 137,000      
Scenario, Plan [Member]            
Income Taxes [Line Items]            
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent         21.00%