Investments (Details) - USD ($) $ in Thousands	Jul. 27, 2019	Apr. 30, 2019
Short-term investments:
Total held-to-maturity and short-term investments	$ 163,634	$ 150,487
Long-term investments:
Total long-term investments	4,887	9,386
Held to maturity securities
Long-term investments:
Total long-term investments	2,194	9,386
Held to maturity securities | Municipal securities
Short-term investments:
Total held-to-maturity and short-term investments	7,873	5,332
Held to maturity securities | U.S. government securities
Short-term investments:
Total held-to-maturity and short-term investments	81,689	63,205
Long-term investments:
Total long-term investments	1,006	7,404
Held to maturity securities | Corporate bonds
Short-term investments:
Total held-to-maturity and short-term investments	74,072	81,950
Long-term investments:
Total long-term investments	1,188	$ 1,982
Available-for-sale securities
Long-term investments:
Total long-term investments	2,693
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments	$ 2,693