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Investments (Details) - USD ($)
$ in Thousands
Jan. 25, 2020
Apr. 30, 2019
Short-term investments:    
Total held-to-maturity and short-term investments $ 148,502 $ 150,487
Long-term investments:    
Total long-term investments 26,409 9,386
Held to maturity securities    
Short-term investments:    
Total held-to-maturity and short-term investments 135,302 150,487
Long-term investments:    
Total long-term investments 21,462 9,386
Held to maturity securities | Municipal securities    
Short-term investments:    
Total held-to-maturity and short-term investments 7,565 5,332
Long-term investments:    
Total long-term investments 15,887  
Held to maturity securities | U.S. government securities    
Short-term investments:    
Total held-to-maturity and short-term investments 45,439 63,205
Long-term investments:    
Total long-term investments   7,404
Held to maturity securities | Corporate bonds    
Short-term investments:    
Total held-to-maturity and short-term investments 82,298 81,950
Long-term investments:    
Total long-term investments 4,558 $ 1,982
Held to maturity securities | Certificates of deposit    
Long-term investments:    
Total long-term investments 1,017  
Available-for-sale securities    
Short-term investments:    
Total held-to-maturity and short-term investments 13,200  
Long-term investments:    
Total long-term investments 4,947  
Available-for-sale securities | Investment in limited partnership fund    
Long-term investments:    
Total long-term investments 4,947  
Available-for-sale securities | Variable rate demand notes    
Short-term investments:    
Total held-to-maturity and short-term investments $ 13,200