Investments (Details) - USD ($) $ in Thousands	Jul. 28, 2018	Apr. 30, 2018
Short-term investments:
Total held-to-maturity and short-term investments	$ 118,303	$ 113,649
Long-term investments:
Total long-term investments	36,712	40,656
Held to maturity securities
Short-term investments:
Total held-to-maturity and short-term investments	118,303	113,649
Long-term investments:
Total long-term investments	36,712	38,514
Held to maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	14,964	35,344
Long-term investments:
Total long-term investments	2,031	2,046
Held to maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	33,514	31,620
Long-term investments:
Total long-term investments	28,809	27,356
Held to maturity securities | Corporate bonds
Short-term investments:
Total held-to-maturity and short-term investments	66,825	46,685
Long-term investments:
Total long-term investments	5,872	9,112
Held to maturity securities | Certificates of deposit
Short-term investments:
Total held-to-maturity and short-term investments	$ 3,000
Available-for-sale securities
Long-term investments:
Total long-term investments		2,142
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments		$ 2,142