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Stock-Based Compensation - Schedule of Fair Value of Share-based Awards Measured with Weighted-Average Assumptions (Detail)
12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Risk-free interest rate 2.08% 1.38% 1.84%
Expected life of the options 6 years 3 months 6 years 3 months 6 years 3 months
Expected volatility of the underlying stock 82.30% 80.90% 85.10%
Expected dividend rate 0.00% 0.00% 0.00%