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Share-Based Payments - Valuation Assumptions for Stock Options (Details)
3 Months Ended
Jul. 31, 2018
Jul. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term in years 6 years  
Risk-free interest rates 2.82% 1.98%
Volatility 84.40% 87.10%
Dividend yield 0.00% 0.00%
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term in years   6 years