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Equity Method Investments (Details)
3 Months Ended 9 Months Ended
Jan. 27, 2018
USD ($)
Jan. 28, 2017
USD ($)
Jan. 27, 2018
USD ($)
Jan. 28, 2017
USD ($)
Jan. 31, 2019
JPY (¥)
Jan. 31, 2019
USD ($)
Apr. 30, 2018
JPY (¥)
Apr. 30, 2018
USD ($)
Dec. 27, 2017
JPY (¥)
Dec. 27, 2017
USD ($)
Schedule of Equity Method Investments [Line Items]                    
Equity method investment activity, net of tax $ (418,000) $ (8,000) $ (418,000) $ (119,000)            
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%   19.00%              
Equity method investment activity, net of tax     $ 418,000              
Carrying value of investment $ 1,503,000   $ 1,503,000              
HAPSMobile | Expected                    
Schedule of Equity Method Investments [Line Items]                    
Payments for purchase of interest         ¥ 209,500,000 $ 1,900,000        
SoftBank                    
Schedule of Equity Method Investments [Line Items]                    
Ownership percentage                 95.00% 95.00%
SoftBank | Expected                    
Schedule of Equity Method Investments [Line Items]                    
Payments for purchase of interest             ¥ 150,000,000 $ 1,400,000