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Investments - Auction rate securities (Details) - Auction rate securities
$ in Thousands
12 Months Ended
Apr. 30, 2018
USD ($)
item
Apr. 30, 2017
USD ($)
item
Investments    
Number of available-for-sale securities | item 2 2
Available-For-Sale Securities    
Amortized Cost $ 2,250 $ 2,700
Gross Unrealized Losses (108) (203)
Total $ 2,142 $ 2,497
Minimum    
Available For Sale Securities    
Maturity period of available-for-sale securities 1 year 1 year
Pre-determined interval to reset interest rates to current rates 30 days 30 days
Maximum    
Available For Sale Securities    
Maturity period of available-for-sale securities 16 years 16 years
Pre-determined interval to reset interest rates to current rates 35 days 35 days
Available-for-sale securities | Maximum    
Available For Sale Securities    
Period for which the issuer of securities is not required to redeem the securities 365 days 365 days