Investments (Details) - USD ($) $ in Thousands	Oct. 26, 2019	Apr. 30, 2019
Short-term investments:
Total held-to-maturity and short-term investments	$ 161,635	$ 150,487
Long-term investments:
Total long-term investments	13,478	9,386
Held to maturity securities
Long-term investments:
Total long-term investments	8,531	9,386
Held to maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	21,761	5,332
Long-term investments:
Total long-term investments	5,015
Held to maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	65,826	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	74,048	81,950
Long-term investments:
Total long-term investments	2,495	$ 1,982
Held to maturity securities | Certificates of deposit
Long-term investments:
Total long-term investments	1,021
Available-for-sale securities
Long-term investments:
Total long-term investments	4,947
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments	$ 4,947