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Investments - Auction rate securities (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Jan. 28, 2017
Apr. 30, 2016
Available-For-Sale Securities    
Amortized Cost $ 2,700 $ 3,100
Gross Unrealized Losses (282) (320)
Total 2,418 2,780
Auction rate securities    
Available-For-Sale Securities    
Amortized Cost 2,700 3,100
Gross Unrealized Losses (282) (320)
Total $ 2,418 $ 2,780
Auction rate securities | Minimum    
Available For Sale Securities    
Maturity period of available-for-sale securities 2 years 2 years
Pre-determined interval to reset interest rates to current rates 30 days 30 days
Auction rate securities | Maximum    
Available For Sale Securities    
Maturity period of available-for-sale securities 17 years 17 years
Pre-determined interval to reset interest rates to current rates 35 days 35 days
Auction rate securities | 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