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Related Party Transactions (Details)
12 Months Ended
Apr. 30, 2018
USD ($)
Apr. 30, 2017
USD ($)
Apr. 30, 2016
USD ($)
Dec. 31, 2017
Dec. 27, 2017
JPY (¥)
Dec. 27, 2017
USD ($)
Jan. 31, 2017
Long-Term Incentive Awards              
Revenue $ 29,597,000            
Unbilled related party receivables 3,145,000            
Ownership percentage             49.00%
Design and Development Agreement              
Long-Term Incentive Awards              
Maximum net value 75,789,000            
Revenue 29,597,000            
Board member | Consulting agreement              
Long-Term Incentive Awards              
Amount paid to related party $ 48,000 $ 80,000 $ 96,000        
HAPSMobile              
Long-Term Incentive Awards              
Ownership percentage 5.00%     5.00%      
Payments for purchase of interest         ¥ 210,000,000 $ 1,860,000  
Unbilled receivables and retention | HAPSMobile              
Long-Term Incentive Awards              
Unbilled related party receivables $ 3,145,000            
Ownership percentage 5.00%