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Investments - Amortized Cost and Fair Value of the Auction Rate Securities (Details) - USD ($)
$ in Thousands
Jul. 29, 2017
Apr. 30, 2017
Amortized cost of available-for-sale securities by contractual maturity    
Total $ 2,250 $ 2,700
Fair value of available-for-sale securities by contractual maturity    
Total 2,053 2,497
Auction rate securities    
Amortized cost of available-for-sale securities by contractual maturity    
Due one through five years 250  
Due after 10 years 2,000  
Total 2,250 2,700
Fair value of available-for-sale securities by contractual maturity    
Due one through five years 251  
Due after 10 years 1,802  
Total $ 2,053 $ 2,497