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Investments in Companies Accounted for Using the Equity Method - HAPSMobile (Details)
12 Months Ended
Apr. 30, 2018
USD ($)
Apr. 30, 2017
USD ($)
Apr. 30, 2016
USD ($)
Jan. 01, 2019
JPY (¥)
Jan. 01, 2019
USD ($)
Apr. 17, 2018
JPY (¥)
Apr. 17, 2018
USD ($)
Jan. 27, 2018
Dec. 31, 2017
Dec. 27, 2017
JPY (¥)
Dec. 27, 2017
USD ($)
Jan. 31, 2017
Schedule of Equity Method Investments [Line Items]                        
Ownership percentage                       49.00%
Equity method investment activity, net of tax $ (1,283,000) $ (119,000) $ (138,000)                  
HAPSMobile                        
Schedule of Equity Method Investments [Line Items]                        
Ownership percentage 5.00%               5.00%      
Payments for purchase of interest                   ¥ 210,000,000 $ 1,860,000  
Option to increase its ownership percentage               19.00%        
HAPSMobile | Expected                        
Schedule of Equity Method Investments [Line Items]                        
Payments for purchase of interest       ¥ 209,500,000 $ 1,900,000              
Other assets, long term | HAPSMobile                        
Schedule of Equity Method Investments [Line Items]                        
Carrying value of investment $ 2,020,000                      
SoftBank                        
Schedule of Equity Method Investments [Line Items]                        
Ownership percentage                 95.00%      
Payments for purchase of interest           ¥ 150,000,000 $ 1,407,000          
Equity method investment activity, net of tax | HAPSMobile                        
Schedule of Equity Method Investments [Line Items]                        
Equity method investment activity, net of tax $ (1,283,000)